Something to think about
1. in winter very few fish are aggressive compared to summer fish
2. of thoes aggressive fish a smaller % of them will act aggressively towards a fly compares to summer fish.
3. aggressive fish willing to take a fly and are selective to size or pattern are even yet a smaller % of the small % we are already talking about.
With this in mind I think size matters only to the point that the fly is visible to the fish. There may be an exception where a fish will respond to one fly but not another but thoes I think are very very rare. I think such exceptions are rare in fact extremely rare.
Everyone has their pet theories abuot flies and why fish take them but one thing is true. None of us really have an accurate clue let alone an understanding of why fish take a fly. I have found that effective fishermen are effective regardless of the fly they fish. I believe it's about presentation and determination.
Only steelhead matter!