The first step in creating any 3D object in SketchUp is to create an enclosed 2d object. By enclosed I mean a shape that has a perimeter and an area. That creates a shape with a face. Every flat face in SketchUp can be “pushed” or “pulled” into a 3 dimensional shape. For example, if you want to create a 5″ cube you would first draw a 5″ square. Then use the push/pull tool (keyboard shortcut P) to select the face and either push it backwards or pull it to the camera for the desired shape.
To determine the dimension of your push/pull command you can either drop it off wherever you click or you can specify a dimension and press enter on the keyboard. You can also modify that dimension after you have completed the push/pull. For example, lets say you want that same 5″ cube but accidentally typed in 4 for the push pull dimension and pressed enter. There’s no need to back up and repeat the push/pull command. Instead, without selecting another tool or moving the camera angle, type in the correct dimension and press enter again. You can also repeat this process to visually see how different dimensions will look. Press 5 then enter and now the 4″ push/pull will modify to 5″. Want to see what 10″ will look like? Type 10 and press enter. Nope, that was too much. Lets stick to 5″. Type 5 and press enter and you are back to where you wan to be.