We have 365 Business Basic which I believe gives our users a 50GB mailbox
A number of them are about to hit the 50Gb limits and we would like to add a Archive facility for them.
Could some confirm what license we would need to add a archive for a user and also if there any limit to how big a archive mailbox can get to?
You will need an Exchange Online Plan 2 license, or any SKU that bundles it. With it, you're getting 100GB quota for the primary mailbox and unlimited quota for the archive.