Welcome to the forum
I LOVE hostas, and have about 10 now (I've been a little hosta crazy - just started my garden this year) Sum & Substance is one of the larger hostas that will grow about 3' x 3'. You want to make sure you plant
it far enough away from the tree
so that it has room to grow that large
As Donna said you want to watch the water since hostas do like their water
You might think about building a rasied bed around the tree
about a foot or more away from the trunk to be sure your hosta is getting enough water. Good luck!