You could get away with 23s.. but there's certainly no harm going bigger - it will help with grip, comfort, etc. You need tight cages for sure. It's more of an issue with Paris-Roubaix but Flanders will still chuck your bottles around. Metal doesn't automatically mean tight. I lost a bottle from an expensive metal cage in Paris-Roubaix.
You could get away with 23s.. but there's certainly no harm going bigger - it will help with grip, comfort, etc. You need tight cages for sure. It's more of an issue with Paris-Roubaix but Flanders will still chuck your bottles around. Metal doesn't automatically mean tight. I lost a bottle from an expensive metal cage in Paris-Roubaix.