Sure, maybe both Jack and Rose could have fit on that board. However, floating objects can only hold so much weight without sinking. Together, they would weigh up to 300 pounds, and possibly sink the board. So it could have fit both of them, but not necessarily hold them both. So stop bitching about Rose, amirite?

I know right, I think this every time I see that stupid statement, it's like, dude, it's a plank of wood, how buoyant do you think it is?
Plus have you ever tried to climb out of water onto something that's floating that already has someone on, chances are it would tip over and Rose would fall off, it probably took way too long getting her on there in the first place.
Also it's not like she forbid him from climbing on, he wasn't all like "Hey, I think I'll get on this makeshift raft too" and she was all like "REJECTED!", it was his decision to let her have it.