Bobby, Oxwall checks with the php.ini directive of your server>>reads the value for max upload file size>>pulls this value and inserts it into the " (Server limit X Mb)" line.
On my Oxwall installation I have: (Server limit 32 Mb).
>>>EDIT: Oh yeah - and why was the field set at 2.5 upon oxwall installation but when you change/save something on that admin page, you are forced to change it to the *supposed* upload limit of 2 Mb?
Oxwall's default is 2.5Mbs.
"2MB" value is pulled from your server settings.
This is done for your convenience. So that you don't have to go and compare values .
You are not allowed to exceed the value pulled from your server settings, because even if you would have been able to change the value in admin panel, this would have caused errors on the front end of your site when users tried to upload files that exceed server's max. upload size limit.
Hope that this helps.