depends what type of engine you are talking about. I have some Champ decals for steam engines. There are 2 sizes of heralds: 5 ft x 4 ft the other is 3 ft by 2.3 ft. These decals are for large engines, northerns, hudsons, etc
My Microscale HO decals measure 4 ft by 3 ft. as used on smaller tenders such as switchers and mikes.
Hope this helps.
RAY