Im assuming youve got a 1991-98 non-16v/Willy pumped up arches Clio. If that is the case, then you can go up to 17s - but as Juss will tell you, itll slow you down.
15s are probably the best on a non-big arch Clio as you get rubbing without arch mods with anything bigger. I know! I used to have an old 1.2 without big arches - 16s rubbed like crazy, 15s were better.
15s will allow you, with low profile tyres to stay near to the original rolling radius (i.e. total circumference of the original 13" wheels and tyres) - hence the effect on speed will be minimal.