British Columbia: Root size- Espalier Apple and Cherry

I am planning on planting 2 espalier trees against my fence where I have a 3 foot wide retaining wall the runs the length of my fence. It is about 1.5 feet high. The apple tree is on a mm26 rootstock and the cherry in a colt -stock....my question is how big will the roots get? It there enough room in the wall for them to spread?

 

Basically, the roots will be a mirror image of what is growing above ground.

 

The previous post is probably right when it comes to most non grafted trees, however most of the espaliered trees are grafted on a variety of rootstocks. Look for dwarf or semi dwarf rootstock for a more suitable size to grow against the fence.

 

Is mm a misprint? I have "m 27" as espalier apple rootstock. I don't see "colt" in the cherry section
Are the trees already espaliered? If you are on a larger rootstock, be diligent with your pruning so that
what is growing above ground is mirrored in the size of the roots.

 

mm26 is a semi-dwarf, i skimmed over that part in my first post. Not sure what kind the colt is.

 

My understanding is that semi-dwarf are alright to use as espalier.