If you are using start parts then one option is to place a sketch in the start part to scale the model. This could be a sketched line, circle or box. When starting a new model set the controlling dim of this sketch to the "size" desired. The model "size" is dynamic and changes based on the features created. By placing a sketch in the start part you can adjust it to the longest length of the part design you will be creating and it should yield the desired result. You can always delete this curve from the model once you have a real feature in the model.
I have never needed to do this and have built parts measured in microns up to several meters in size. You can of course scale an entire sketch inside sketcher if needed. You should also use absolute accuracy by default if you are not already.