Buggy "func_physbox"
The mass scale is not only for the falling speed the box has, it should be working to let you grab it. Did you decrease the mass scale when you increased the func_physbox size from 16x16u to 32x32u? Try for example 0.01
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Yes, I have even tried 0.0001, still the same problem :/
You are obviously doing something wrong. Could it be that you create the func_physbox too close to a surface like the floor and it gets trapped/stuck in it?
I'm attaching a test, check and let us know what were you doing wrong, ok? 
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[/spoiler]Well, your's works, I have no idea why... well thank you 
It works in any map I create, exept in my test map... It is very old and I did very stange thing in it ^^. Go to bin, test map !
You could post your test map so we can help you find the problem 
This is like a puzzle unsolved, I sincerely think a thread should be closed (or abandoned) when the issue has been found, never before. It can allow ppl to revise each of the main properties to consider in order to get the proper settings, don't you think?
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[/spoiler]Well if you insist, here is my test map ! I managed to recover it
http://www.mediafire.com/?rwz7rdvqhf9d7sw
Also, i did not mentionned that, but i can't pick up the panel too. haven't tried in another map.
Thank you for your help
Like I mentioned earlier, the problem seems to be the texture - only that my suggestion to try a wooden texture sucked. At least the wooden one you tried doesn't work. However, when you apply the metal one you have used on the smaller boxes that you can pick up, you'll see that you can pick up the bigger box as well.