Well it could be another issue not addressed yet.
But anyway you need to decide how big a file to allow by user group.
Then the entry is in bytes.

So I assume you are a admin and to allow a admin larger file sizes.

GOTO Control Panel » Group Management
scroll across to the right from administrators and click on Forum under edit permissions.
Maximum size of gallery images
Note the max size in the note section which is the max size set by your host.
If this forum is a gallery forum, this setting dictates how large each image may be. Max size set in php.ini is xxM, so you may not exceed this size.

Enter a value in the gallery forum or forums in bytes.
So to allow a 1mb file the value would be 1048576

And select update.
Now if you desire to upsize other groups just do the same for them

Also while you are there another field called
Total # of gallery images
will limit the number of files they can upload at a time.

We also have some limited nfo on the WIKI at:

Blue Man Group
There is no such thing as stupid questions. Just stupid answers