Valve stem seals advice

’87 1.8 8v GTI cabriolet DX engine with K-jet

I have two mk1's 1.8's and I have just bought a 2nd hand head for one of them as the old one was warped and in poor overall condition.
The seller advised I change the valve stem seals as they are old and I have been told there are a few different types/sizes …didn't want to order incorrect type and waste time.
Could any one advise please as this will be my first attempt at changing a car engines seals, I have managed it on my bandit motorbike.
Numbers on the replacement head are 026 103 373 AA
Thank you.
**update** I have worked out it's 8mm and have sucsessfully fitted them.