For sizes when you have you tape;
4ft x 2ft = 48in x 24in = 121.9cm x 60.9cm ... but obviously you can go larger if you have the space/finances!
So the two storey cage you are looking at is twice this size (it is 120cm x 53cm, but then two levels) but as a rule, only one floor level is counted because, for example, if your piggies never went upstairs then there is no point counting it as floor space because it is never used.
The cage looks great, but I would make a side to the ramp for safety, but it does look steep so they may not use it. I was thinking that if I get more piggies in the future, it would be a good space saving option, but I was going to block off the hole for the ramp and us it as two cages for seperated piggies.
For a cheaper version you could go for
this one. I know that a few people on here have meationed that they have this and is good value. I haven't ordered from zooplus myself but I've not read of anyone having any major issues.
EDIT! Actually, please ignore that link, I've just noticed that you are in Ireland and they don't deliever that particular cage to Ireland for some reason? I'll leave the link in though incase someone in a county they will deliver it to browses through. Sorry about that!