So I’m having this wierd problem and I’m not sure if I messed up the option before or not. I created a mesh and made it into insertmesh brush, I turned on the curve mode and tested it. The curve looks fine but no matter how big my brush size is ( The red circle one ) it’s still remain small. I tried to modify the original mesh and checked if it had the correct polygroups to make it as a curve brush but it still had the same problem. Not sure what to check next 
Your mesh just might be really small. Usually, before you make the mesh an InserMesh brush, you should click ‘Unify’ in ‘Tool > Deformation’ to make it the ideal size.
I did click unify in deformation but it still the same small size.
Check your Document size, maybe it’s too small.
Can you zip the brush and upload it here?
Ah it’s the document size! Thanks for the help guys 
I got it back to normal now