Hi light_bulb,
Unfortunately Osclass can not do that right now (but in the next version will work on that!).
The problem is that, to check the size of the file, the file has to be on the server, so it has to be uploaded first.
Other solution is to check the size of the file ON the client size (browser), to do that you will need adobe flash or a HTML5 capable browser. Flash is widely available, but it's being used less and less each day, also, there're some configuration (flash version + operating system) where it doesn't work fine. HTML5 penetration is slow, and again some browser may support HTML5, but not all the features, so it's not an easy task.
On mobile devices (phones, tablets) it's even more difficult. Most of them don't have flash support, and HTML5 support is very very limited.
So the solution is to use HTML5, if not available Flash and if not available normal upload. Which is a lot of work.
We've crated an issue sometime ago, you could follow the development here :
https://github.com/osclass/Osclass/issues/60Hope you understand