Lemon tree leaves falling

I've been given a lemon tree which last year produced 3 lemons.  Two of them are still on the tree.  This year the leaves are falling off though it's still producing flowers.  It's in an 8 inch pot  and about two feet tall.  I wonder if I should re-pot it.

I would be grateful for info on what compost to use and what size pot.

Best wishes