The size of cattleya flowers depends somewhat on the size of the plant. A cattleya with 20 pseudobulbs will generally have larger flowers than a plant with 3 0r 4 pseudobulbs.
Light is a more important factor. Stronger light will usually produce larger flowers.
Water should not be an issue unless the pseudobulb that is blooming is severely dehydrated. If the blooming pseudobulb is very wrinkled then the plant isn't getting enough water and the flowers will be smaller. This is seldom a problem though, unless the plant is severely dehydrated, because blooming pseudobulbs steal water from the backbulbs.
I'm going to get a lot of negative comments about my third suggestion. Try using bloom booster occasionally during the fall and winter (I start using it at the end of October). Plants need phosphorus to produce flowers and some of the new fertilizer formulas are low on phosphorus. Many growers will disagree but I seem to get more flowers and larger flowers with bloom booster. Good luck.
Last edited by tucker85; 01-06-2013 at 01:14 PM..