Um ... so, normal FOV, right?
Navigate yourself over to Documents -> BFBC2 -> settings.ini.
You should see a "Fov=XX" value somewhere in there. If not, you'll have to manually add it yourself. Bear in mind that FOV values determines the vertical FOV, as opposed to the traditional horizontal FOV (as found in a certain popular shooter). You'll want to use
this to determine your desired FOV. For example, if you'd want a horizontal FOV of 85 (like me), then plug it into the calculator and you'll find your magic number to be about 73. Thus, your entry in settings.ini should read "Fov=73".
Edit: Otherwise, if you're just talking about changing the size of your gun on the screen, I'm not sure if that's even possible ...