bgolpmp,
The wire has to be strong enough to support it's own weight, whatever size that happens to be, for whatever type of metal it is. That depends on how long of a span it will be, and how many 'spans' you can manage, and how 'droopy' each span would be. The strongest 'small' wire I can think of is 'CopperWeld', a copper clad steel wire. I think it comes in 18 gauge? It isn't invisible, but isn't that easy to see once it's been up for a while, it tarnishes.
A 'long wire' or random length wire requires a good ground system to work properly. A ground rod is very seldom even close to a good ground system. Laying several lengths of wire as radials will do much better. Those radials should be as long as possible, probably the same length as the wire in the air, times as many as you can put down. They don't have to be very deep at all, just enough so that you don't trip over them or wind them up in the lawn mower.
Long wire, or random wire antennas are not going to be 'super' usable for multiband antennas. How well they work is a function of their length just like any other antenna. Using a tuner is almost a certainty, the better the tuner the more bands the thing will be capable of tuning. The 'smaller' the tuner the closer that antenna's length will have to be to some multiple of a wave length for the band of choice (fractions of a wave length too, not just whole wave lengths).
Not quite as simple as usually thought, huh?
- 'Doc