The 5.1's will take up to 2.5 inch tyres. The 4.2's up to 2.35.
I was looking at these wheels for a new build that I've now put off for a bit. I found this on the DT swiss site. There's no info on the Hope site regarding max tyre width.
I've been using Hope ProII's on Mavic EX321's. They are OK as a doitall wheel set.
When I say doitall I don't mean ALL! I wouldn't do full on DH on the things but they will take a fair sized tyre for my Cotic Soul.