No residuals can be calculated from a rectification that uses only 4 points (the minimum number) so for confirmation of your measurements you should have at least 5 points for each image to be rectified, and more is better.
Perfect measurements on a perfect plane with perfect point placement in images of infinite resolution will show zeros in all columns but in the real world close will have to do.
If your measurements are bad all you can do is eliminate points or re-measure but if your measurements are good, high residual values will probably be the result of poor point placement in the source image.
In this case select the point with the largest residual value and move that point in the direction opposite the value (for +Y move the point down, for -X move it to the right, etc.) then rectify again (F5) to see the changes. Once all the values are about equal the results will be as good as it is going to get with the points you have.
The residuals are presented in real world units so the maximum error in the rectified image should be the same as the largest residual.
For a discussion of the Z deviation see the section on 3D coordinates.