I have a giant Avocado tree in my back garden (in London N17) grown from a seed, which I literally threw out of the kitchen window. I can't remember when. However, two years a go it bore 12 fruit and none since then. It is quite bizarre, but how do I encourage it to bear again and how do I curtail its growth? I had two other 5' ones but chopped them down. Now there are two more growing which I would like to keep small.
You can do some judicious pruning, no more than 1/3 of the branches at a time. The 2 small ones can be turned into houseplants if you chose or transplant to another location. In order to bear fruit they avocado trees need pollinators. You might want to consider planting flowers that draw bees and other similar pollinators to encourage your tree to bear more fruit.