I have full size flexisoft prosafes and they fit my very fine ISH standing at 15.3hh, so probably not too generous. But they do also fit my chunkier 16.2hh ISH.
I used full size on my old 14.2hh Connemara, so not hugely generous. Is Fabio very chunky? They fit my 17hh ISH perfectly as well, he's fairly average build.
We've got the Eskadron Flexisoft Pro Safes as well and have had them for a couple of years now. The front ones fit our 16.3hh warmblood mare just fine, however, the fetlock boots may be a bit on the small side. Saying that though, she does have quite big joints and it can generally be a bit difficult to find boots that fit her. Really depends on what type of horse you want to buy them for.