Depends on the appliance doesn't it? Go to the appliance spec on the manufacturer's website and figure out the distance from the bottom of the appliance to the OPEN door of the oven. That should be no more than waist high on the primary user. Say it is six inches from bottom of oven to top of open door and primary user is 5'4" (average height for women in the US). Waist is at about 2'9"...subtract 6"....Bottom of oven opening is 2'3" above floor. 4" toe kick...ergo cabinet below is 19" +/-. Two deep drawers or a double door cabinet with tray storage.

The look is of course important, but maybe you should consider the functionality first, especially for a kitchen. As you say that you have a bad back, probably the wall oven would be a good choice - and it can look beautiful with the right design, if you have enough space. Please find attached an example (handle-free model Essenza by CucineLube kitchens)

Wall: a lot easier to access oven when installed at bench top height. Wall: more options being width, height, combi, features etc. Wall: easy to get induction top of your choice.

How about a lift-up cabinet to keep the microwave in? That way it won't look out of proportion when matching a 900mm oven with a much smaller microwave on top. All the best!