So you're purchasing and incubating meat bird eggs, or a dual purpose breed and using them as meat birds. What breed are you planning to get? That does influence the answer some, as far as size of birds goes.
By 5-6 weeks, for most breeds, it will be obvious if you have males or females based on comb size and color. For production birds, you may be able to tell as soon as 3 weeks.
If you're going thru Murray McMurray hatchery, they don't sell meat bird eggs. They do have some nice selections of meat bird chicks, and some appealing combination packs, if you want meat and eggs, you might consider that one.
Males taste like females until they go thru sexual maturity. That happens around 12-15 weeks and older for most breeds. Cornish cross, or cornish roasters should be processed when indicated by the hatchery, so generally 7-12 weeks or younger. They will be tender that entire time, but size of the bird will determine best way to cook it. For roasters, generally I like my CX no older that 7-8 weeks, as after that the size difference between the breast and legs and back is great, so I part out the carcass for more even cooking, and take off the breast meat.
Males and females taste about the same as far as stock is concerned, when you're using the carcass and bones.
For dual purpose roasters, anything up to about 14 weeks appeals to most people. For fryers, 10-12 weeks or younger is best. Whatever breed you get, process them over a range of ages to determine which flavor you prefer. For best results make sure to rest the bird in the fridge until the joints move easily before cooking or freezing it (you can rest after you thaw it out after freezing it, but you must make sure the joints move freely before cooking or whatever bird you cook will taste like rubber).
Anything over 16 weeks I pressure cook to be sure it's tender. Rest in fridge until joints move easily, then pressure cook. I'm bad about letting my birds go long - if you have anything besides CX or CX adjacent breeds, they can handle a bit of delay. 3 month roasters (ginger broilers, red rangers, etc) can be processed whenever. However, CX MUST be processed when recommended or they start dying on you due to heart and leg issues. Even then, 2-3% loss is normal when raising CX.
Egger males, or heritage breeds, which includes most dual purpose breeds unless the strain is specifically optimized for meat (check out Freedom Ranger hatchery dual purpose meat bird offerings), will have much less meat than 3 month broilers or CX. CX has the best meat per cost of raising it, hands down. But they can be tricky to raise, and you have to buy chicks every year - they respond differently than every other breed I've tried to my raising practices. Egger roosters - you'll be lucky to get 1-3 lbs meat off the carcass for most breeds that mature to 5-7 lbs live weight. Many folks are fine with this.
I tried a bunch of different meat and heritage breeds to see what I liked best as far as processing age, size of carcass, and reproducibility. So far, my favorite is the Freedom Ranger Hatchery strain of New Hampshires. They're not super great at eggs, but they lay enough to raise more, and they are huge and chunky, and make a nice big carcass. About as big as they can be without having CX issues from it, IMO.
