40mm, something like this.
https://www.screwfix.com/p/floplast-solvent-weld-waste-pipe-white-40mm-x-3m/44310
I have had 3 HDMIs, 1 optical and 1 aerial coax through it with no problem. No need for a draw string, I have had everything out of it whilst redecorating and then just fed it all back...