Having heard so much about how raiding two hard drives speeds up their operation I was wondering whether it is best to fit one large drive or two smaller drives and raid them.

When you say "raid them", I presume you are talking about mirroring (I don't know that you could get a speed boost with concatenation).
Mirroring may speed up reads though not writes (perhaps slightly slower writes), you also get the security of if one goes down then you still have your data. Software RAID won't be quite as efficient as hardware RAID, so that's something to bare in mind too. Proper RAID controllers are expensive...

I'd go for two drives, as big as you can afford and mirror them.

Thanks for your reply 'Forsquare'

My question was for raiding two drives into one drive which I believe is not the same as 'mirrored'. I don't know the terminology for the other options.
I intend to use the bios to setup the raid as my brother has done on his computer. Unfortunately he did not take any note of whether he got better performance or not as he only raided to increase size for two ssd drives.

You mean striping, RAID 0.

Yes, they are faster, but not twice as fast, and if one drive dies you lose the lot; as data is spread equally across the array.

You can go for a mirrored stripe, RAID 0+1, for data safety, but that requires four drives.