The only way to know for 100% sure the sex is to vent sex your birds. This sticky
has detailed instructions for doing that. Don't feel bad if you can't vent sex even with such good instructions, because I can't do it.
Sexing by size is useless, for several reasons. Especially at a young age. Generally hens are larger than roos, but I have light hens and heavy roos.
When will I have a good idea which are the roos if ever before they "crow"?
Other than vent sexing you will have to wait till they crow. That can happen as early as 4 1/4 weeks, but late bloomers can take a lot longer than that and you may never catch them crowing unless you separate them from the rest of the birds. I built a special pen for this, which then doubles as bachelor pads. Before that, I just used a plastic tote to do the roo test.
This time of year you will have to supply at least 14 hours of light a day, or the roos won't crow, and the hens will not lay.
How old before I need to seperate them from eachother?
That depends on pen space , roo to hen ratio, the temperament of the birds, and/or the planetary alignment of the solar system, or the price of gold.
Generally speaking, 1 roo to 3-4 hens, and about 1 square foot of floor space per bird. I use 2 ft. by 2 ft pens, so I calculate things based more on temperament of the birds. 1 roo to 3 hens in 4 square feet is optimal. 1R+4H=Fine. I can over stuff 2R+6H in 4 Square feet, but, not for the novice or faint of heart.