Keeping things to 4' increments is good. Worst case, try to keep it to 2' increments.
I'm not going to download the HD planner just to see, but does it allow you to do layout?
Grizzly Tools had a workshop planner -
https://www.grizzly.com/user/shop-planner - but Adobe discontinued Flash, so they're still working on the next generation of it.
It would allow you to chose Grizzly tools and lay them out in a space, but you could also make boxes to represent other things - ie: a Suburban is approx. 224.5" x 80.5" or your tire machine is 4' x 3' - make a box of those dimensions and label it.
Old school way was to cut pieces out of graph paper (a square is one foot by one foot) and move them around on another sheet representing your garage. I'm sure you can find other software to use as well.
In any case, lay out the sizes of your vehicles and equipment and spend some time moving them around on paper to see what fits.
In any case, once your garage is built you'll find that it is too small and you'll wish it was larger...