Surfing Waves

Hello, so I'm fairly new to surfing and the other day I noticed my fin cracked pretty bad. I just don't know what size replacement fins I would need to get. I found a pair on the internet that were a little bit smaller than the ones I currently have. I just don't know if they will fit in the slots or not. I have three fins and a 7' board. Thank you so much ahead of time!

We can't gauge your fin needs well , because you have given us no pictures or description of either the fins or the fitting on the the board into which they go! There are many fin systems, without pictures we are guessing

If it's the Green M.Dolsey board you posted before, it should be equipped with FCS fins. Get something around the M-5 size. Yours probably came with black plastic G-5s originally, but maybe the first owner changed them out. Height of the fin is around 4.5 inches. If the ones you found on the internet are about 4.5", it'll be fine. Another thing is to go to a local surf shop. They usually have a Onezees box, full of mis-match single fins. Just replace the one you "cracked".

Yes sorry its for my green Michael Dolsey Board, it had white fins on it before which had have like the two slots for each fin, if that makes any sense.

They most likely FCS fins.
So you use an Allen key on the grub screws loosen them, take the fins out, they obviously have two lugs that fit and match into the plugs.
There are a number of fin makers that produce FCS type fittings , take a fin with you and see how you go at the local surf shop!