I see you have an
ECO 13.
Don't know if anyone else noticed.
If your
ECO has anything like stock equipment,
it likely came with a 20A cord or less.
A 12/3 extension cord might actually be
plenty more than needed
.
Length within reason is not much of an issue since
there is really not much voltage drop on 110AC.
Really should know what equipment you run
in order to give realistic advice on which cord
is best suited to your needs. Of course "too heavy"
will work fine even if it might be inconvenient at times
or a bit bulky.