you do a lot of math.
if you want to figure out measurements for using 2 1/2" strips...figure they are 42" long...after trimmin the salvages you should probably figure 40"...so you want a king size quilt...110"x110"...so first 110 divided by 2" (your 2 1/2" strips give you 2" finished strips..the 1/2" is seam allowances...so you would need 55 strips to make the width...then you need 55 more to add to length making it 80" wide...then you need another set of strips...so if you wanted to just use 2 1/2" strips it looks like you need...55+55+55 or 165 strips.
see lots of math...using graph paper makes it easier sometimes to figure it all out, then you can draw it out to scale