There are plenty of discussions on the subject. Basically if it's over 60' you won't be able to go on the Leeds & Liverpool (except Wigan to Liverpool) or the North-Eastern waterways such as Calder and Hebble, Huddersfield Broad (the Huddersfield Narrow is OK), Sheffield etc.
Otherwise you can go nearly everywhere with a boat up to 70'. A bigger boat measn ... you get a bigger boat, and usually more feet per £. Pro: it's bigger so more room inside. Cons, it costs more to licence or moor, and finding a big enough mooring can sometimes be difficult.
Personally I'm glad we have a bigger boat - me and Pingu can spend the whole day more than 60 feet apart - and if I want to go on the L&L again I'll just hire a boat up there.