Can I reduce the width of a board half an inch by sanding?
It is not feasible to sand down a wooden member by a whole half of an inch. 


If something is too long then cut off the additional half inch using a saw. 
If something is too thick (wide) by the half inch then rip saw off that extra thickness or use a planer to remove it.
If you cut something too short you will have to go get a replacement piece and re-cut that to the proper length.


You have learned a valuable lesson here. Plan out everything in advance carefully by making sure you understand what is needed and materials are available.