Hey, I have a 99 Discovery I
I have 245/75/16 right now with the stock alloy rims that came on the defender as well.

I'm not sure what size tires I can buy.
Do I need to keep the width at 245 or it won't fit into the rim?

 

245 is your height, 75 is your width

 

your rim size is 16x7, the recommended largest size tire that you can put on your rims is a 255/75r16 (tall 33). i run 285/75r16 (wide 33). there are a lot of defender guys who run the 305/70r16 (very wide 33). so it all depends on what you are trying to accomplish, whether that be axle clearance, looks or practicality. do you have a lift? if so what size? do you wheel? what type of trails? basically what ever tire that you want to buy, you can buy