Here is the breed standard directly from the page of the Canadian Pembroke Association - gives you their weights too.
Size
Moderately long and low; well balanced. The distnace from the withers to the base of tail should be approximately 40 percent greater than the distance from the withers to the ground. Height (from ground to highest point on withers) should be 10-12 inches (25-30cm). Weight, in show condition, the preferred size dog of correct bone and substance will weigh approximately 27 pounds, with bitches approximately 25 pounds.
Here is the link to the full standard:
http://www.pembrokewelshcorgis.ca/standard.htm