Pot size for outdoors

So getting my stuff ready for next month.
Im growing in Ireland and will be growing in pots as the soil where my plants will be is crap.

Problem is that a 50l bag of plagron all mix is 20euro.

I wanna put out about 20 plants but cant afford to give each plant a 50l bag.

What size pots do you think are suitable for a decent, big yeild.

Would 25l pots be sufficent?
Thanks

the bigger the better, to get that root mass huge!!!remember, the bigger the roots, the bigger the fruits!!happy growing!

At least 10+ gallon pots,... better yet, just get some kickass soil and mix it directly into the ground with a tiller.

You need pots as big as you can get. Or you can do that thing that Closetgrowth said

The pots size is only applicable with the amount of good soil i can get.

The place itself is very overgrow but i am considering getting this soil for tomatoes and digging it into the groung.

Anybody ever use this or know anything about it

http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=10288043&fh_view_size=10&fh_start_index=0&fh_location=//catalog01/en_GB&fh_search=tomato&fh_eds=ß&fh_refview=search&ts=1270597911787&isSearch=true