I have and still do both options (short and too long Heaters)
IME the "too short" option may take a little longer to clear the dew as it might dissipate in a normal circle pattern with a small spike sticking out where the
Heater is missing. This "spike" will clear, but be slightly larger than the circle pattern.
A 3" overlap is going to add almost double the heat to that 3" portion, however I've never noticed an issue and it seemed better to me than having a gap.
I ended up contacting Dew-Not and asked for recommendations on 6” vs 7”. Like you mentioned, they thought that depending on weather conditions, some dew may form due to the gap, if I went with the 6” heater.
They suggested making a custom size so there’s no overlap or gap, and had it in the mail to me the same day. Fits perfectly!
Excellent customer service from Dew-Not.