17cm is for 434 MHz.
Right, it should be 17. My bad.
I've also read 13 in some guides but it should be 17 for 434MHz and that's what I'm using.
315 MHz ~1.5ft,
1/4 of that, ~11cm, should work well
What is 11cm? 1/8 wavelength?
Guess it should work too.
Experiment a bit. Try 11-12 cm and route it around inside your enclosure and try 24 cm and make more loops. It doesn't necessarily have to stick out.
Maybe it has to if you use a metal enclosure.
Any piece of wire should do. I've used thick single core copper without plastic insulation, thin single core jumper wire with insulation, a multi core wire with insulation from an ethernet cable (there are eight of these in each cable) and that worked fine too. I don't know what's best in theory.
The alarm devices you're mentioning are probably using coil antennas or pcb loop antennas which are a bit harder to get right.
Maybe someone here can tell what dimensions you would need for that.
What kind of range are you aiming for?