Size isn't the issue its the frosts. They are tender and frost will kill them. In Yorkshire I wouldn't plant them out until end of May early June. Personally I'd grow them on in pots until then and then transplant them. I often have to move mine up a pot size in late April early May to keep them happy.

Hi, I agree with Sbg, if they weren't tender, you could plant them straight into your garden, but they are, that's why they have to be started off inside, Derek.