There is no search option for this at all, as far as I'm aware, so this can only be done by extracting the search results or sorting through them. What I mean by user friendly is a nice user interface, a simple to use drop down menu. In contrast, it would not be user friendly to have to type minfilesize:"2MB" or similar in the search box.

If there currently is no search at all then there should be a search at first (and as one aspect it should be user friendly). I asked because mentioning "user friendly" in the task summary felt unneeded - everything should be user friendly when possible...
As you wrote "multimedia search by file size" does that also include videos? What's the usecase for videos?

That would only be for pictures/images. I said 'multimedia' because search on wikipedia is divided between 'content pages', 'multimedia', 'everything' and 'advanced', and it should of course only be an option for a 'multimedia' search. I'll clarify the title.

Yann: Do you plan to work on this? The Priority value should reflect reality but does not cause it...

I am not developer. The priority value should match what is useful to people, contributors and readers.

[offtopic] @Yann: Everything is "useful" if you just ask the right subset of people. :) Anyway, Low and Lowest probably does not make any difference if no-one volunteers to work on this task. And for the records: https://www.mediawiki.org/wiki/Phabricator/Project_management#Setting_task_priorities