I met a very large F5 male once.. probably 14" at the shoulder and a good 20+ lbs. My F5 is 6 months old and 8lbs and I am going to guess between 10 - 11" at the shoulder... so he probably will just be the size of a normal cat. I am HOPING he will get bigger, because I love the giant cats as well, but you really just never know.
The only way I think to have a guaranteed large cat is to get an F1 or F2. I think even some F2 females are on average nearly regular house cat sized, so I would definitely go with a male Savannah of any generation you end up picking.
It will also be a good idea to perhaps purchase your kitten from a breeder who is doing a repeat breeding of a previous litter so you will already know how the other kittens grew up, and hopefully your chances of getting what you want will be increased... but still there are no guarantees, you just can't guarantee genetics!
Just keep in mind about the earlier gens as well, the higher Serval % yes will give you a more exotic look, but also some temperaments can be trickier to live with or will require much more training and patience on your part. With that said, I am sure there are F1s and F2s out there that are just sweet as can be and totally calm and well behaved, but on average you will have a much higher than normal energy cat. Even my F5 is very energetic and is either running around, cuddling with me, or eating or sleeping. My Ragdoll cat spends much more time lounging around than my SV, although I do think the SV is calming down more with age... he is actually starting to sleep more often haha.