It shouldn´t be to small so that you have enough space to work on and can lay big pieces of latex out on it, mine is approx. 155cm x 115cm. (Don´t forgett a role of latex has at least a width of 90cm!)
The thickness of the acrylic wont be a big issue, since you wont be able to cut it all the way through. So its basicly up to you and that what you can effort.
Plexiglass used in that way, wont last forever! So keep in mind that you will have to buy after quite some time a new one or that the first one you buy is very thick so that you can just polish the cutting scratches off when it becomes to much.
Where to buy? In regular home improvement stores! (HomeDepot, Bauhaus, OBI, Silvan etc.)
Acrylic is pretty hard to work with, because it is so brittle. So measure your workspace exactly before you go out and buy it, so that you can get it cut exactly to your needs. You dont want to cut it yourself.
TIPS: Before you put your latex sheeting on, smoothen the edges of the plexiglass and maybe put some strong tape around the edges, so that the latex sheeting doesn´t get stuck there or rips.