It should be written on the bottom of the tank I think, but as my largest Paterson is the 2x135 size I think you simply extrapolate from 290ml per 135-sized reel.
If it fits in with your chosen developer and dilution (regarding volume of dev per film) then you might also look at the Jobo tanks which require only 250ml per 135-sized reel. The Jobo tanks are also modular in design, which might prove convenient. They do pour less quickly though, so that should be considered in cases where a short dev time is required and you are trying to empty a litre or so of developer speedily.