I get an OutOfMemoryException (in VisualStudio 2010) when trying to load  large ecw images  (15 - 67MB). Is this a known feature?

Small files (<1 MB) load inconsistently; some are fine but on others I get an Overflow error.  I think this is something to do with the files themselves and how they have been created but I have not been able to work out  what the problem is.

I think you're right. See http://dotspatial.codeplex.com/workitem/22214.