From what I've read, 16 gauge should be sufficient for 10A at 75-100ft. For more than 10A, use 14 gauge for upto 50ft and 12 gauge for upto 100ft

In the days before many of the gasoline-powered yard tools, a fellow bought a electric hedge trimmer and used it with about 400 feet of extension cord. It soon overheated and eventually burned up. So he bought a heavier duty trimmer and it burned up even sooner.
If in doubt, check the voltage at the tool while it is under load. Probably will require two people and some knowledge of electrical test equipment. Anything less than about 112-115 volts is asking for trouble with the tool.

Invoking my little southwire voltage drop calculator, it would appear 14G is sufficient for voltage drop for 10A or 100 feet.

But bus is right, running motors on too low voltage can cause real problems.

Thank you. I'll get a 14ga if we go 100 feet, and 16ga will be fine for 75 feet?

"NOTE: When operating cordage close to it's rated ampacity be sure it's in free space. I had one coiled up on a reel to run the freezer in the garage. I found that it had permanently stuck itself together in the coiled shape."

Ron is correct, however, the coiled up extension cord - whether on the ground, or on a reel, is now an inductor and will heat up (and may melt) very quickly under load.

It's not as much an issue with "free space" to cool the wire as much as it is a need to eliminate the inductive effects caused by the extension cord being in a coil.

Explain to me the inductive effects of two wires with balanced and opposite currents in it. Further, if you even took ONE of the conductors and looped it around a cord reel, I suspect you'll find the change in impedance marginal. The problem is indeed as I specified it.

how many feet do you have to edge? I have a B&D hedge trimmer that runs quite a long time on a single charge...you'd be amazed. If you get an edger then you have no cord! Just be sure you get the better charger where you can lv the battery on the charger all the time without it hurting the battery.

Well, I suppose a 50 foot cord could reach most areas, but I would feel better with 75 feet.

It's a 120v, AC Only, 10 AMP weed eater. IDK watts, again, I'm not very electrically inclined, lol.

The thing most likely has a universal motor on it and 10A is most likely the most it draws (it might actually draw less, there's a lot of hype on Amps and HP on small appliances like this at times).

But even at a lesser current, the heavier cord will probably still be a good idea for the reasons I mentioned in my first reply.

Be aware that stranded conductors have a bit more voltage drop than do solid conductors of the same gauge. And use of the cord eventually results in some of the strands breaking, effectively reducing the gauge of the conductor.
Bottom line is that flexible cords should never be undersized for the load.

The AMPACITIES I reference were from the tables for cordage and already take the stranded nature into account.

You must be careful on the length of extension cord on electric Weaver due to the obedience in the line it will cause your weed eater to possibly burn up if you have too much impedance therefore restricting the current flow to your weed eater

I would get a 12 GA, 100 feet. They are relatively common and not too expensive. What kind of cord does your weed eater have? Also get one of these devices which holds two cords together.