Q.when is the best time to repot dwarf apple trees
I have two dwarf apple trees in 12-inch wide by 11-inch tall pots. They are about 4 ft tall and in flower. Can I repot them now and what size pot should I use?
Assuming that you want to pot up, in other words put into a larger pot to promote growth, use a pot one size larger than the current one. From a 12" pot go to a 14" diameter. This article has general information: https://www.gardeningknowhow.com/edible/fruits/fegen/dwarf-fruit-trees-a-planting-guide-for-fruit-trees-in-containers.htm
The "when" to repot is a subject of debate. Some people say late winter - early spring, while the plant is still dormant; others say late spring - early summer, when the plant is heading into its period of greatest growth. The dormant period is recommended to avoid damaging roots, but I prefer the summer time, because bonsai growers have proven that cutting roots is not damaging to the plant, so that's not really a factor.