A first glance in the Farnell catalog makes my eye fall on one which is 45x12 cm... So that's space enough I should think. Don't take one so big it gets unpractical though. Remember that long lines are not advisable, and that is what you will inevitably get on a big board. So keep it small. A very common standard size (at least in Europe) is the Eurocard size, which is 10x16 cm. That is a very realistic size to prototype on. They are very cheap.
If you're smart, you'll go for the slightly more expensive fibreglass (white-ish) type material with SINGLE PADS.
Fiberglass is tougher than resin bonded paper (the brown material in the picture I mentioned earlier). There are also types with STRIPS (where all adjacent pads are connected into one long strip of copper). These cards are not very convenient to work with, because you have to cut the strips all the time to isolate groups of pads, which is not fun work. If you consider a carreer in circuit design, get used to prototyping boards, because you'll be seeing a lot of them.