I asked:
What are the standard dimensions of a typical ICW day marker?

Answering myself, FWIW to you folks, the Federally-maintained ICW day mark
smallest sizes are designated 3SG (3-feet on edge, Square, Green) and 4TR
(4-feet on edge, Triangle, Red), for up to one nautical mile range. Sizes
4SG and 6TR, and 6SG and 8TR are also used for more widely-spaced beacons.

Tom