What size should I do?

Hey there,
Right now i've got a Corn Snake \ Dumeril's Boa and Ball Python, I want to bring more snake's and Geckos and also try to breed some :)
First I want to build a snake rack and I don't know what size should I build it and the size of the cage to begin with.. I read at almost every care sheet on the net the for example the adult Ball Python need at least 36 inch long cage yet at some famous YouTube channels its look like smaller plastic cage on the racks..

How can I tell if its the "right" size and how much should I put in every species to begin with?

You will find most people use the CB-70 (made by IRIS) or equivalent tub size for adult ball pythons. These tubs measure approximately 33.5 x 17.7 x 6.0 (L x W x H). It is impossible for me to argue you can't keep ball pythons in these tubs, because there is a plethora of evidence to the contrary. However, I do think these tubs are too small for a permanent enclosure. They have too little floor space for larger specimens (you can't really fit an x-large hide in there) and they are only 6" tall, which is very little head room. As a quarantine tub, CB-70 tubs work well. Regardless, many commercial rack systems exist for CB-70 tubs.

I would encourage you to try for a cage stack of PVC or other plastic enclosures. You will find most people have great success with, and encourage the use of 48" x 24" x 12-15" enclosures. Assuming you are gunning for a 36" or 48" size, they really don't take up much (if any) additional space. For example, a reptile basics CB-70 rack is 34.5" Deep x 19.25" Wide x 30.75" Tall (61.50" if you stack two) and any rack you might build will have to be similarly sized. A stack of (5) 36" x 24" x 12" cages is 36" x 24" x 60" with no stand. You for sure lose "volume" of snakes per unit area, but you have much more appropriate enclosures.

If you are dead set on a rack setup, animal plastics has VE-175 tub racks they use for larger constrictors. These tubs are 52" x 20" x 12", which might be a bit overkill, but there are not a great deal of tub sizes between these and the CB-70 unfortunately. If I was going to do a rack setup, I would build one designed for these tubs or just buy one from animal plastics.

For BP you are looking at 32 quarts tubs for males and females under 2000 grams

For females over 2000 grams you are looking at 41 quarts tubs

For corns (males and females) you are looking at 32 quarts tub (different rack obviously since temp requirement are different)

Those are appropriate size enclosure and the standard.

I keep my juveniles in the 32 quart tubs, but I house my adults in 55 quart under-bed tubs which are almost TWICE as large floor space. One 55 quart tub takes up an entire 3 feet wide rack shelf, while I can fit two 32 quart sized tubs on one shelf. (so 2 juveniles per rack shelf, 1 adult per rack shelf. The Rack shelves are 36" x 26" floor space.