Grey tit at my new bird home ︎

Hi. My friend has made me a beautiful bird box. I've recently seen a grey tit taking interest and pecking at the entrance hole. I've just measured it and it looks around 26-27mm. Is this wide enough or do I need to make it a little bigger?

When do they start nesting? I have plenty of feed in the garden and wouldn't want her to miss out on using the house.

Thsnks in advance Nicky