U.S. Executive Order 10834 specifies proportions for the flag, as recreated here.

The U.S. flag has a proportion of 1:1.9.

Each stripe is 1/13 the width of the hoist and each star is 4/5 the width of a stripe.

The union is the width of seven stripes and 2/5 the length of the fly. //

Shown here is a timeline of each official iteration (including notable variations).