I've got fairly broad shoulders and chest for a man my height & weight, but I can get my forearms completely together from wrists to elbows in order to make a passing platform.
Of course, I'd never actually pass a volleyball that way.
As manzi says, get the thumbs & wrists together and your arms should make a pretty ideal passing platform without alot of forcing. If your body or arms are so bulky as to prevent that, maybe you need to look into resolving that before you work on trying to force your forearms together.
