Line weight for reels

[Buanán]

All I wanted to know is - would an Orvis battenkill No 1, rated as maximum line #4 cope with the #5 line I use with my Orvis #5 rod?

Simples.

A.

If you lost some of the backing, most probably. 

[Wildfisher]

Just cut off 10 yards of the running line like Alex said. You don't need it, better to have more backing just in case. Fly lines are far too long for  most fishing and for most casters.

[Allan Crawford]

Could you take it to a tackle shop and try it out for size and balance ?