Darling, that’s the fun part of being a writer! See it in your mind’s eye, and then describe what you see:
“The gnarled bark of the tree’s trunk formed a series of steps leading upward into the branches, and there, hidden among the green leaves and growing apples was a house. Small and somehow invitingly cozy, it looked like the sort of place everyone’s favorite grandmother might live, if she lived in a tree, of course. The windows were open, and a hint of cinnamon teased the senses, invoking the warmth of apple pie and spiced tea.”
You don’t need to measure it out in feet, or meters. Let the reader use their own imagination, and just give them the general shape of it.