Ensign size used to be, as a rule of thumb, 1 inch per 1 foot of vessel. On modern yachts however this is found to be a little small for a vessel to be considered "well dressed."
Roughly speaking a 3/4 yard Ensign should look right on a boat around 21-26 foot
- 1 yard for 27-34ft
- 1-1/4 yard for 35-42ft
- 1-1/2 yard for 43-50ft
- 1-3/4 yard for 51-60ft
- 2 yard for 61-70ft
This is just a rough guide and some discretion and personal taste may need to be applied.