Replacement battery

I own  mk 2.5. It is fitted with a 015 battery.

All manuals and guides say it should be 063 which is somewhat wider. As the battery clamp is a fixed width type, I am wondering best course of action.

It definitely needs replacing, I assume it is lead acid not gel but how would I know.

It is a second car left outside and only used occasionally.

as ever your thoughts and help are very much appreciated. 

i would add that 015 batteries are not straight off the shelf