I'll have to dig out the torch to check tip size but if memory serves it looked less than 1mm, I just dialled in the recommended current from the manual (in the box with the leads) and it cuts nice.
I did make my own bent wire height guide to make it easier and quicker when using a length of timber as a rule for cutting 2m x 3m x 3mm plates into manageable 2m x 1m sections for storage.
The cutter is rated to 12mm with the supplied torch and there's no mention of using bigger tips, i think swapping to the standard tip size supplied with your machine's OEM torch will solve your issues,
Regards,
Nick