The best way to go would be to take measurements of your current best-fitting suit and compare them with those of the Isaia suit you're interested in. The key measurements that determine fit are (a) in the jacket: shoulders (sleeve seam to sleeve seam straight across the back), chest (just below armpit to armpit doubled), waist (at waist button--probably second of 3 or top of 2), length (collar seam down to hem at back), and sleeve length (shoulder/sleeve seam to cuff); and (b) trousers: waist and inseam for sure, along with width at knee and cuff (if possible). The latter two measurements don't bear much on fit, but on fullness of the cut. (You might find the Isaia trousers slimmer than you like; not likely fuller.) I know that STP don't give much in the way of measurements with their suits, but I imagine that if you contacted their sales reps or customer-service people, you could get them to give you these measurements. You could have them do this for both their 52 and 54. I know that if I were ever interested in an STP suit or jacket, I wouldn't purchase until getting this information some way.