That one is up to you, some peps go on week 3, some go on week 4, some go on week 5, etc etc, but when you do go it will double in size sometimes treble, its really up to you.
I haven't seen them double in size although I hear people say that a lot. Your yield will be higher the longer you let it veg. I'd let it go another 2 weeks if I were you.
I let the plants have as much as I can, and how long I veg depends on how much I'm growing. If it's under 5 plants il veg up to 8-10 weeks to allow 4 size and a good yield. If I'm doing 20 I will only veg for 4 weeks so I can get a quicker turnover of bud.
I agree with Illegal Smile in that I don't think they double in size, but I'd definately say an extra 50% wouldn't b exaggerating.