a regular size visine bottle will do it, unless you think you mignt need more piss then you can fit in that, in which case go for the biggest. im sorry if its poorly explained but its very hard to explain. basically what you are trying to do is fill it up the first time, but now you have some air still in the bottle which is wasted space. so you hold the bottle upright, squeeze it until there is no air left in the bottle, and then quickly suck up the rest of the pee into it. and then you have a full visine bottle of pee..how wonderful!