Here's some info I came across and put in a word document:
Batting and Mattress Sizes
Below are the common sizes of quilting batting made by most manufacturers. Keep this handy reference when purchasing batting for your quilt.
Batting Sizes
King -- 120 x 120
Queen -- 90 x 108
Full -- 84 x 96
Twin -- 72 x 90
Baby -- 45 x 60
Craft -- 34 x 45
Below are common sizes of mattress. The measurements are of the top of the mattress. The thickness of the mattress can vary so the size of the quilt will need to compensate for the thickness of the mattress.
Be sure to add the mattress measurement plus the "drop" on both sides to compensate for the thickness of the mattress.
Mattress Sizes
Twin -- 39 x 75
Long Twin -- 39 x 80
Full -- 54 x 75
Long Full -- 54 x 80
Queen -- 60 x 80
King -- 76 x 80
California King -- 72 x 84
Instead of one border that may have to be very wide(not sure how that would look) how about 2-3 borders?
As to how small you can get away with, it depends if you want overhang or not. Any overhang is a good thing but I've had occasion to use a quilt that just fit the top of the mattress. Not ideal but this was over another quilt that did overhang (top one was for extra warm). So looking at the Cal. King mattress size you can make the quilt somewhere between this size and the size of the batting - don't forget you need excess batting on all sides.
FYI - on King size beds the longer measurement is the width of the bed. This is important to know if you use directional fabric.