Nieves,
As a rough guide on size for molds, here is what I find about right for most of my cheeses:
For hard / aged cheeses (cheddar, gouda, etc.), a 4.5" diameter mold is good for up to 2-gallon makes; 5.5" or so for 3-gallon makes; 6.5-7" or so for 4-gallon makes. When you get up to an 8" diameter mold, you really need 6 gallons or more to get the right proportions.
For camembert/bries, you want a thinner cheese so that it will ripen all the way to the center before the outside is too ripe. So, for example, I spread a 2.25 gallon make into three 5" molds, and that seems to work well.
I have a combination of bought and home-made molds. For the cam/brie, I've used plastic Cool Whip containers with holes drilled. This is not the ideal shape, but it worked reasonably well. I have a 7" diameter mold made from an HDPE plastic bucket that formerly carried ... wait for it ... drywall joint compound. Yes, the thought is a little off-putting, but the type of plastic was perfect, it was sturdy, and it was easy to clean thoroughly. It has worked quite well for my 4-gallon makes. Again, drilled holes for drainage.
Some people drill holes in PVC drainage pipe and use that. I have seen mixed opinions on this -- some people seem perfectly happy with it, but others feel that there could be some unpleasant chemicals that can leach from the PVC over time.
I look forward to pictures on your presses, and on your cheeses!