I had a male that was 6 inches with a female that was 3.5 inches and they would breed constantly they were also bonded so I never had to separate them so to answer your question he isn't to small
The problems that can arise is they won't be compatible which is always a big possibility with flowerhorns and the other thing is your male could be fertile or infertile but you'll never know till he tries breeding
All in all there's no reason not to try
Put the egg crate divider in the mild of the tank then leave them like that for a week after that you can try 1 of two things for the best odds of seeing if this pair will work
1 cut a hole in the divider just big enough for him to go threw but the female can't go threw ( usually it's the other way around because the male is usually larger but because of your case it will have to be for him ) so they can interact but if she starts beating on him he can go back threw to his side to get away
2. after a week let them in with each other and see how they cohab if they get to aggressive separate them for another week and try again
In my opinion option 1 always works best but it doesn't hurt to try both they could cohab great and bond together but with flowerhorns it's unlikely
Good luck
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