How thick does my object have to be in order to be strong enough?
The minimum thickness of your object depends on the material you use.
With white plastic, a wall with a thickness of 0.03 in (0.8 mm) would be reasonably strong but flexible. Above 0.08 in (2 mm), the walls are really strong.
With coloured material, minimum wall thickness is 0.12 (3 mm) in to ensure your object is solid enough.
With silver material, minimal wall thickness is 2 mm. The model should not include holes less than 3 mm diameter. The object should not include any moving parts.
Keep these values in mind when you design and order your objects. In particular, be careful when you change the object scale.