Please have a look at the Organize Your Messages by Using Filters article. You should be able to set up a filter based on file size (IIRC this includes both email body and the attachment) and make it do several things with it, one of them being adding a label to the message.

The filters are nice, but they dont solve my current query. I can easily define a filter to say, star messages greater than 900kb.

I use stars to identify other attributes, this will confuse me. Which stars apply to the attribute, which ones are greater than 900kb?

Is there a way to modify the size column and have an identifier right there in that column?

As you wrote you wanted to assign a label, I thought you meant a new/special one, which seems the best way to do and better than using stars allright. Note that you can (and have to) create a new label called, let’s say, "> 500KB" first, give it a special color etc. in order to use it for a filter rule. You can create one using Message - Tag - New tag (or add the menu button for it by Customizing the menu bar). Note however that the actual labeling is done for new messages from then so don’t expect all existing ones to get labeled, but you could do so manually. I just tested it for a new message and it works, showing either a file size in the Tag column or the entire message line appearing in red for instance.

If you rather want to add a new item for the columns, that would probably be possible as well (like anything in Mozilla products), maybe by using some add-on. Is your intention to have larger emails appear with e.g. a red size to distinguish them from smaller ones, or have an additional label (like Size) to have its own column? (Admitted, my locale uses "label" for both tags and labels, so that may cause confusion here.)

Is there is an option to tweak the format of the email size column config editor?

In other words, make a rule within the config editor?

Please answer contributor's questions first, not by asking new questions instead. Basically your original question has been answered and you already have your solution.

Sorry, I beg to differ. My original question:

When scrolling though messages, the KB and MB labels in the size column look similar. In order to reduce my inbox size, I want to assign a label to any message that exceed a preset size (such as 900KB or 500KB). This will visually identify large message that I can target for deletion or attachment extractor.

How can I do this?

I quite sure this question is unsolved.

Contributors suggestion of adding a label or starring the messages is good, but due to obvious reasons and immense possibility to create confusion, this option is not viable. In summary, my question remains unanswered, the contributor offered a simplistic solution that adds unnecessary complexity and is viable.

I am asking a very related questions, not a new question. Can I add the characterizers such as *** or ^*^*^ to the values shown in size column?

No. But you can tag the messages, and you can create custom tags and custom colours. And you use filters to automatically assign the tags. Or just move the offending messages into their own folder. Or use a Saved Search folder to create a virtual folder of oversized messages.

There is no provision that I know of to add arbitrary "labels" .

You have the right to differ, however

- I didn’t suggest to use stars, you came up with that - I excused myself for possibly mixing up labels and tags (or labeling and tagging) above - you did ask a maybe related, but new question instead of answering mine on what your intention is in the 2nd paragraph of my 2nd reply

To answer the latter myself, obviously you do want to use a new label, i.e. add a Size column so that you have 2 Size columns based on large or small messages and only have the new one displayed, and you don’t want to use the Tag column. As said, add-ons for adding labels may be available (or not) but there is not much sense to it - technically TB knows of sizes as one entity so you have to rely on criteria and hence filters for any user customization/prerequisite, or your wish would be similar to adding a 2nd Sender label based on alphabetical surnames for instance. That leaves you to use the option of adding a custom tag based on your wishes and use the Tag column for a yes/no indication, like Zenos confirmed above.

My problem is NOT solved.

I will find a solution to my original question.

And when I find it, I will post it here for the "so called" Guru's to see.

Thank for the responses so far, but these do not solve the visibility issue I am trying to address.

Tonnes said

You have the right to differ, however - I didn’t suggest to use stars, you came up with that - I excused myself for possibly mixing up labels and tags (or labeling and tagging) above - you did ask a maybe related, but new question instead of answering mine on what your intention is in the 2nd paragraph of my 2nd reply To answer the latter myself, obviously you do want to use a new label, i.e. add a Size column so that you have 2 Size columns based on large or small messages and only have the new one displayed, and you don’t want to use the Tag column. As said, add-ons for adding labels may be available (or not) but there is not much sense to it - technically TB knows of sizes as one entity so you have to rely on criteria and hence filters for any user customization/prerequisite, or your wish would be similar to adding a 2nd Sender label based on alphabetical surnames for instance. That leaves you to use the option of adding a custom tag based on your wishes and use the Tag column for a yes/no indication, like Zenos confirmed above.

I DO NOT want a second size column, I want to modify the existing size column. Why do you think the add-on "SIZE in KB 1.0.2" exists? It partly addresses my query, not fully.