Chocolate cake

If I don't have the size of cake tin specified in my recipe, will the size I do have be OK?
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Toby is 2 tomorrow and he wants a big sticky chocolate cake for his party! The problem is that my recipe specifies a 20cm round tin with a depth of 7.5cm, but I haven't got a 20cm round tin.

I do have a 23cm round tin, and I've also got a square tin, which has a side length of 15cm and a depth of 6 cm.

Which one do you think would be best?  Or should I go out and buy a new cake tin?