You will have to read the labels on the trees in a garden center.  The final size will be determined by the root stock they are grafted onto.  The label will give you the final mature size of the tree.  Your crabapple tree is close enough to provide pollination for another apple tree.  You need to check the final height and width of the tree(s) you select.  Allow for enough room for the branches to expand.  Some trees get vary wide and there will only be room for one.  My Honeycrisp was approximately 20 feet wide when mature but was only 12 feet tall.  It will not work to underplant your tree(s) as the area will be shaded and you need to walk under and around your tree to bag the young apples, prune it and harvest.  It would be better to plant just one tree and use the other space in that area for your rhubarb, etc.  FYI blueberry shrubs need approximately 4 years to be fully mature.  https://extension.umn.edu/fruit/growing-apples