It's more of a mechanical issue than an electrical one. 600W is less than 4A RMS into a 50 ohm load, and it
doesn't require a very big wire to handle that - #22 probably would work.
Soft-drawn copper wire (solid or stranded) only stretches when it is under sufficient tension. Stretch might be an
issue if you are using it to guy your mast at the same time, but there is no need for that much tension on a dipole
otherwise. (You'll never get it perfectly straight anyway.) Even on 160m I rarely pull up more tension on the wire
than I can easily pull with one hand - probably 10 to 20 pounds at most.
Stranded wire is more flexible than solid, and that can be better when the antenna is blowing in the wind.
However, if you get solder in between the strands of the wire it becomes more brittle than solid
copper. (So I tie the wire to an insulator to take the strain, with a pigtail that isn't under tension to make my
CopperWeld is quite strong - that's what I'd use, for example, if I had to cross a street or parking lot where
there would be other people underneath. It is a pain to work with, however, like a big spring. Note that
real CopperWeld is something like 30% copper, so it has a good thick layer of copper.
Stranded CopperWeld is not as good of a choice, especially for 80m and 160. That's because the
copper coating is applied to each individual strand, so the layer is thinner than for a single conductor. As
you go lower in frequency the skin depth gets deeper, and once significant amounts of RF start flowing through
the underlying steel the losses increase.
Don't confuse real CopperWeld with copper-plated welding wire - the copper coating on the latter is made as
thin as possible, exacerbating hysteresis losses, and it can corrode off quite quickly. A friend put up an antenna
using such wire and it worked for one night before it became an aerial dummy load.
For a permanent antenna, a spool of #12 or #14 insulated house wire, either stranded or solid, will about as well
as anything else unless there is a safety issue, in which case use CopperWeld. For a portable or temporary that
you are going to be putting up and taking down frequently, something like Flex-Weave makes sense, though I get
by with #18 to #24 stranded, insulated hookup wire, as it is easy to work with. (The PVC insulation decays in
the sun, and I have to replace the smaller sizes every 2 years or so here in Oregon because the insulation
contributes a significant part of the strength of the wire. But that isn't a problem for antennas that I set up
a few weekends a year.)