5-6mm should be perfect for such a large bed, i run 6mm on my 300mm dia Delta bed and it’s very satisfyingly solid. Just make sure your heater is powerful enough - i’d recommend at least 1kW or you’ll be waiting forever for it to heat up.
@Jeff_DeMaagd I was using a 450 watt 12V power supply running at 36 amps. I went ahead and timed it to see how long it actually took to heat up. Its actually around 6+ minutes to get to 60C.
I’ve not timed mine, but with my 500w supply and the bed bang bang off the 12v mains, I’d say it gets to 60c in 5 min or less. Not bad. I routed a channel into the middle of the underside of the aluminum bed and wired my thermistor up in there to try to get the most accurate reading possible, it’s not touching the heating element in any way.
Thank you so much, I am not so good with everything about electricity. I will try with 4mm first. If it’s not work well will i can remove easily the silicon patch to glue on other aluminum plate or it will be lost?
Did someone ever try something like this:
@Michel_DA_SILVA transferring a silicone heater to a new surface can be challenging, as the adhesive will not come off in a reusable state (and is somewhat hard to remove completely). Some cheaper heaters might even get damaged when you pull them off.
@Thomas_Sanladerer thank you, so i have no choice i can’t do a mistake 
it’s à bit more presure… so i will choose 5mm