Draw Rectangle draws a constrained rectangle with or without dimensions in your sketch.
Draw Rectangle command is found from the Draw panel (Sketch tab in the Ribbon menu):
Click in the graphics window to set the center point. If a geometry (eg. line or point) is selected, the center point is constrained with coincident constraint.
Type the Length and Width in the parameter box.
Add dimension can be turned off if you want to dimension the geometry later.
Check the Continue to Extrude box if you want to go directly to the extrude command.
If you want to add parameter names, type e.g. Length=100. Then the parameter name is converted to "Length".