I understand that GA won't provide legal advice and all manner of
disclaimers apply.  Nevertheless, please help where you can:

Is it legal for a tourist to carry a knife in Thailand and if
so, what are the limitations (size, etc.)

I am considering trekking around far from 'civilisation' and want to
carry a 7" fixed blade in my pack/ on my belt and a 3.5" folder in my
pocket.  I will be mostly in the provinces, but may pass through Bangkok.

I took a look and found that you can't carry a knife in a national
park without a permit from a park official:

http://www.krisdika.go.th/lawHtmStaticContent01.jsp?frm=tmp&page=eng&lawType=law2&lawCode=%cd10&lawID=%cd10-20-2504-001

I'd start here:  http://www.findlaw.com/12international/countries/th.html

I'd also recommend calling the US Embassy to Thailand or the Thai
Embassy in Washington DC.