We are thinking of upgrading our gutters from a 5" to 6" but are wondering do we need a 1x8 trim board versus the existing 1x6? We live in Michigan where ice and snow are heavy. Seems there is conflicting info out there regarding this. Thanks.
If the practice in MI is to set the gutters low enough so sliding snow shoots over them (it's not here in KY), then no, a 1x6 fascia is too small. It would also be too small if there were very long runs involved.
The fascia should be sized to the rafter tales. Adding downspouts is one option to get more flow through 5" gutters.