This can be answered through simple ratio and proportion
  
 Let x = width
 y = height
 x/y = x2/y2 = x3/y3
  
 since
 for 18 inches width we have 26 inches as length, therefore
  
 18/26 = 45/y2
 y2 = 65 ft
  
 there is no need to convert the units for the smaller dimensions since we are dealing with definite ratios thus unitless.