Not sure what you mean by "figuring out sizing" Do you mean how to figure out how much yardage of fabric you need or do you mean sizing like in starch??
The Rail fence quilt you linked to is a generous throw size measuring 73 x 85 more than adequate to nap under for either you or your boyfriend but both of you together under the same quilt, I would make it larger. More like 85 x 108 or so. Increase all the measurements given on the pattern you linked to by at least 1/3 of a yard per fabric. I hate how they give you a measurement of "341 inches of continuous binding" , I tend to allow 3/4 to 1 yard for binding for a queen size quilt which is about what you will end up with for a quilt 85" x 108"