Problems creating small models. what size to design at?

I am designing models, currently mainly road vehicles for 3d printing, for T scale models railways (1/450 scale). These models are small - in this scale a short wheelbase Land Rover scales out to just 8mm in length. (some that I did managed to design and print - )

Herein lies my problem - I am not sure what size to use to initially create my designs.

At first I tried using the actual size of the final model, but that failed because the circle tool does not work at those sizes.

Then I tried at either 1000x (this being convenient as I could just convert mm values to m), or at full size of the actual vehicle. When doing this I set the measurement and angle accuracy to the maximum first. The problem with this was when I then used the scale tool to reduce the size of the model to 0.001 or 0.00222 for the actual 3d print size the models do not always scale smoothly and can often end up with holes where some of the faces just dissappear completely. I have tried scaling the model as is, and also converting to a component first, but either way can get the problem.

I would be grateful for any advice from anyone who has experienced this problem, so that I can create my small scale models with curves on the bodywork as required.

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