When given the vertices's of rectangle, we can find the dimensions by using the distance between points formula.
The distaqnce formula if given by :
D = sqrt(x1-x2)^2 + (y1-y)^2
The formula you need is the following:
If ABCD is rectangle given the vertices's, we know that:
AB = sqrt( xB-xA)^2 + (yB-yA)^2
BC = sqrt(xC-xC)^2 + (yC-yB)^2