MTU settings for Vigor 2860

[#1]

Can anyone tell me the correct MTU settings for a DrayTek Vigor 2860 router (connected direct, no BT modem)?
I'm using 1442 at the moment and things are working but is that the highest I can go? I've tried 1458 and 1492 but they won't work.
Can someone confirm that 1442 is the correct setting or do I have a problem somewhere?
Thanks.

[#2]

My 2860n is set to 1492. I think this may be the default as I don't recall ever changing it.
I have not noticed any issues...