Del Brown’s Merkin Crab is an old standby fly that’s been around for years. You can’t beat this pattern for sight casting to permit, but don’t limit this all purpose crab fly to just this species.
Try it out any time you find fish crabbing. Tie Merkins in differing sizes and shades from tan and greens to blue. You’ll be covered for just about every crab variation that crawls around.
Bill Mitchell suggests to fish in sizes #1, #1/0 and #2/0, most commonly #1/0. Picking size is always a difficult decision. Too small might not get seen, too big might spook, and then sometimes big gets the nod when fish are hungry.
You need decent weight, whatever the form it takes: dumbbells, keels, beads etc. This applies to both crab and prawn. The fly has to get down quick and sink in a defensive posture.
Patterns should be imitative or stand out. Crabs can be brown, rust, root beer, blue, olive or tan. Mix and match to copy the local species. Take 3 refusals only on a colour before swapping. Colour is largely academic but there are some obvious hints when you look at flats prey; they tend to blend with the surrounding strata. Starting there can’t hurt.