Thanks pochrist, I appreciate the feedback. Don't be sorry for any 'buts', please. This was written in a few hours and should be considered a draft. Now comes the tricky part which is to test it and adjust it to all situations

For instance, the problem that you point out is mostly likely due to the fact that it was used in a UCS. It zooms by finding the WCS bounding box of each object. Naturally, it has to be converted to the current UCS in order to zoom correctly.
To answer your previous question, it doesn't ask you to select anything because it uses the measurement that you give it to filter out those dimensions that match. It can easily be adjusted to only search in a specified area, though.
By the way, do you see any problems with the units? I'm not used to imperial units so it's much easier to have you test that part.
I'll see if some free time comes up today to build in the WCS/UCS conversions.