@CLmoss This is a Fusion thing you need to Learn (you have no choice in the matter) When Sketching in fusion you don't need to do your measurements straight away, you can added them latter.
Instead of haveing rulers and position cal-louts on screen fusion use's the diamention tool to set the size and position of objects, (I asked the same thing when I started to use fusion) you get use to it and It's faster than a ruler or callouts on screen.
When you go to draw a say a squire you will see the measurements of each side you can set the diamention when doing the squire or you can just wack a squire out and get it ruffly to size and carry on adding your sketch elements.
If you wont to add the diamention when doing the squire the measurement callout that is high lighted is the one you can add a diamention to first, then you can tab to the next one and add the measurement, if the first one is wrong you can just tab back to it.
if you wont the squire to be equal on all sides you can set the first side tab to the next one and just click on the first one it will auto populate the second one.