What wire gauge suitable for small tube amp?

Using an overly heavy gauge will not "suck the life" from the amplifier. As long as the gauge is minimally adequate, the resistance of the wire will be negligible in comparison to the load impedance presented by the speaker, and using a gauge that is much larger than "minimally adequate" will not change that.

16 gauge wire, to cite an example, is specified as having a resistance of about 4 ohms per thousand feet. A 24 foot round-trip run from your amp to speaker, and back to the amp through the other conductor, would therefore be about 0.1 ohms total resistance. That is pretty negligible compared to the impedance of any speaker; using a larger gauge would only make it more negligible.

I'm not saying that resistance is the only factor in cable design that might affect sound quality, and I'm sure you'll have no trouble finding all kinds of opinions as to the sound of various cables, but from an engineering standpoint the performance of the amp will not be affected by using an unnecessarily heavy gauge.

Regards,
-- Al

The problem is in the other direction. I can't tell you the number of times I have seen very long runs of 22 gage wire. You can loose half of the amps power overcoming the resistance of wire like that. The smaller the wire the more resistance, I am not suggesting the bigger the better, your amp will not require a large gage. For smaller wire Mapleshade or Anti Cable give very good performances at reasonable cost.

Yes, putting my previous comment and Stan's good comment a little differently, to be sure it is clear to everyone:

A wire gauge that is adequate to support the run length and the amount of current being delivered to the speakers, as well as a gauge that is larger than that, will result in essentially all of the amplifier's output voltage appearing across the speaker terminals. That happens because the wire's resistance is much smaller than the speaker's impedance (impedance essentially meaning the speaker's resistance at the particular frequencies being delivered, to oversimplify a little bit).

If the wire gauge is too small, meaning the wire is too thin and its resistance too high, then a significant fraction of the amplifier's output voltage will not appear across the speaker terminals, and will appear across the resistance of the wires instead.

Regards,
-- Al