Our apple tree wasn't doing much of anything (it barely even ever flowered), so I finally cut it down and bark grafted 3 different varieties onto it - Liberty, Ashmead, and Cox Orange Pippin. These should all be able to pollinate one another. We also have a Honeycrisp tree nearby that should benefit from this, too.
Full size apple trees get big, I have some on my property that are over 5m tall. There are small varieties that stay under 3m though.
5m is fine. It's going in a spot where I had to remove a 70m pine. I could even put one in my front yard then.