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What size of rare earth magnets should I use on tyranid MC's? For carnifex weapons and heads.

I would like one of the sices to be compattable with the drill from GW.

I used 4mm dia x 4mm deep for my dreads. I used a wood-working bit & turned it by hand. I think the std GW bit is 1mm. I know this isn't the sizes you were looking for. You can get sets of other drill bits quite cheap on Amazon. I would guess you should be looking at 2mm dia at the smallest, & 4mm deep if you can.
This is the company I use:
https://www.first4magnets.com/cylindrical-rod-magnets-c39/2mm-dia-x-4mm-thick-n42-neodymium-magnet-0-16kg-pull-p2560#ps_1-499

GW's pin vice does well enough for me.
I have a 1mm bit in one and a 1.5mm bit. Pilot hole is 1mm, widened to 1.5 for the magnets. Another bit will make it bigger, by hand.

I have 1.5mm diamater x 2mm long magnets for small torsos and bulky arms.
2mm dia x 1mm disks for small shoulders.
3mm dia x 2mm deep for bigger bits, like Space Marine torsos.
I put in the biggest I can fit, to either side of the join.

I have the old GW pin one. I have lost some of the parts, but I stil have the big one that came in it. (Is that the 1MM?)

Most of the things I wanne socket, carnifex arms and heads (not armour plate) have ball joints. I can just cut the joint and glue a magnet on the join and the hole, can I not?

Thanks for the help so far.

Smooth the ball joint off so you know how you want the limb to fit, and then drill straight down into it. Widen the hole and fir the magnet.
You will have made a gap in the shoulder where you shaved the ball down, so press some putty or Blutack into the shoulder to see how far that magnet needs to stick out.