The size of a regular #10 business envelope is  4 1/8" x 9 1/2"

Regular Envelopes

This style, known as Regular envelopes simply means that it is a closed face envelope with no window. They are the most common used in business to send letters, invoices, checks, and other mailings. The #10 size envelope is the most widely used envelope of them all.
http://www.actionenvelope.com/ae/control/category/~category_id=REGULAR



#10 Square Flap Envelopes (4 1/8 x 9 1/2)