In a Poser scene with several models in, how can I select the "body" invisible root segment of any one of those models, so I can easily make the whole of that model bigger or smaller by a set percentage?

Select the figure from the current figure menu, then select body from the current element menus. Then resize the body using yscale (or whatever). You can rename the figure by selecting the body, then object|properties menu and changing the name from figure 1 to something else.

If I understand what you are asking, just click on the figure in the scene, then at the bottom of the document window, click on the down arrow on the right (current element). Then select Body in the pop up window. This is on page 113 in the P4 manual.

Hotkey for that is the "2" key. (Top row, not keypad) Select any part of the figure's body & press 2 1 is current element 2 is current figure 3 is current camera I use these shortcuts all the time

Which of these work with Poser 3?

All of them should. Hot Keys: 1=object, 2=body, 3=camera, 4=light. (Watch out - You're gonna kick yourself for missing these :0 )