The threadcount is what determines the design of the tartan, it doesn't really matter what the size of the sett is as long as the ratios of the threadcount remain the same.

So for example a thread count of 2:4:2:6 would be the same tartan as 10:20:10:30 or indeed 60:120:60:180.
The proviso might be where a designer has restricted it to a very particular size , but I suspect this is unusual

A few years ago I had a client order the , St Kilda tartan but the sett was very small, so I got Dalgliesh to weave it for with the narrowest line to be 1 cm wide. It proved a very successful kilt.

So yes changing the sett size is possible although possibility expensive!