What can I do to increase my flower size?

You need proper ph in that soil ,should be 6.5 and you need to have her near perfect humidity low 30% .Those two along with feeding her the proper nutrients would help to ensure your girls are producing the best fattest buds that they can ...hope that's helps as does what the other growers say there is so much knowledge on this site you'll be a pro in ko time cheers ...

First off all some more pictures would be nice brother. Second of all some information on your nutes and feeding schedule. Then we can try and help. But you need to be feeding them plenty of p and k and if they're still stretching then they still need some n too.

@Elijha420 Hi there mate First of all it says your growing in soil but your ph is at 5.6 When growing in soil ph should be in the 6.2-6.5 to allow all of the nutrients to be available to the plant Also humidity could cause problems with botrytis later in the flowering if it stays at 60 or above Basically just keep feeding your girls a P+K based feed and keep the ph within soil range and you should be all good to get some nice fat buds Happy Growing