I am having big problems modelling a thin tube-intube design in StarCCM+. I want to model the gap between the two long tubes and that gap is only 0.5mm. The length is 250mm. Any experiences with a similar model?

I can imagine that you end up with meshing problems. The ratio between width and length of your geometry is 1:500. A hex-resolution with 20 cells per 0.5mm in the width-length directions gives 20*1*20*500=200000. And still the third dimension is not included.

What is the circumference??

Maybe it would be an idea to only model a part of the full tube (for instance 45 degrees).

You will have to figure out your cell maximum and start from there. Use prism layers (2 or more). Then try with a surface triangle size of for instance 0.025 mm to begin with, then tune it later.