In practice 90% plus used by stove fitters will be 5″ or 6″.
Chimney flue liner (sometimes called flue liner) has a degree of flexibility (not a lot but usually just enough to get around 45 degree chimney bends.
5″ chimney flue liner is usually very easy to slide down a chimney. 6″ chimney flue liner is usually okay to slide down a chimney but it is not unknown to have problems on bends (one may have to break through a wall to assist the liner around the bend). I would never considering fitting a 7″ liner unless I could see the flue top to bottom or know it was large enough.
There are two grades of 6″ liner: 316 grade and 904 grade with 904 grade being the better. See 316 or 904
The brand of 6″ chimney liner I use and I sell from this site can be found here
You can cut 6″ chimney liner with a hacksaw, tin snips or a metal cutting disc on a small angle grinder.
Chimney liner is sold via the metre and arrives in a roll of approx. 1 metre diameter.
Some brands of chimney liner have an arrow printed on the metal. The arrow, if present should point to the sky (it is the direction of the fumes).
As with almost everything in life you get what you pay for. You will find cheap flue liner on the Internet at £15 metre or thereabouts. It really is not worth it in my opinion.