Specifying Rectangle/Width

There are a number of ways you can do that *indirectly.*

First and foremost, you can manually set the dimensions of a rectangle by selecting its layer, then in the inspector, Shape > Geometry > Control Points. Each point is (usually, but not always) represented with respect to a center zero-point reference. So if you need 100 x 100, the control points would be:

-50 50

50 50

50 -50

-50 -50

Once you create that, you can use scale X&Y sliders to specify to size (% will equal pixels) or you can build a couple of Rig sliders controlling the X&Y scale parameters instead (that would be more like the Color Solid sliders.)

A quick and dirty shortcut would be to draw a large rectangle (larger than the canvas) in it's own group (by itself.) Make sure the Group parameters are 2D and Fixed Resolution. You can then use the Fixed Width and Fixed Height parameters to set the width and height of the rectangle. (The group can still be rotated in 3D space but will not interact with 3D groups, so you'll have to resort to "layer order".)

The Fixed Width and Height of a 2D group are "clipping" boundaries, so you're not really resizing the original rectangle, just clipping it to size.

Motion doesn't handle resizing "rectangles" directly because they're not really rectangles. Circles (or ovals) aren't circles and a line isn't technically a line. They are all Bézier "shapes" - just pre-built for you with "modal" variations of the Bézier (pen) tool.