What's a decent size?
based on your stats' 6ft, currently 86kg cheap digital scales say 14% BF.
That's 14% bf. so not 'fat-ceps' . You lose fat on the arms before the gut typically, so at 12- 14% I'd expect you to have a fair bit of definition, you would have a clearly definate horseshoe tricep (even if small). Then 15+ inch arms will look big to the punters.
Having big defined arms is different to having big fatceps. Plenty of fat dudes sporting 21+ inch arms.
You can build big arms through compound movements and avoid issues that arise from grip strength and fore-arm imbalances.
I am guessing I am more than 14% BF , can see most of my abbs (not the lower thanks to pudding doh!) and my
intercostals. Arms stretched out you can see the sepparation in the two
muscles in the biceps, horseshoe etc. My Arms are 18 inches give or take (. Not built by bro curls or isolation stuff.
If you like the isolation movements then you can keep them but, drop the weight and instead focus on squeezing the life out of the contraction on the movements at the top. Focus on crushing the bar handle as you do the rep. If possible use thick bars if available or wrap the bar with a cloth to increase the bar thickness. This wll develop your forearm and grip strength, which will help avoid the tendonitis the bro-curling kickbackers get all the time.