How to print bill over predefined template

Hello, i will try to explain my problem.

I wan to print over predefined template, my client have a bill’s template, and he want to print over there.

The dimensions are: 15cmx20cm, how can i define the dimensions and configure the print job?

With great difficulty at a guess.
Width is doable with the newer table/colum type line expression similar to the reports layout headers but length is going to be your challenge most likely.

You will need to provide an example of the Print before we can offer any suggestions.

What type of Printer is it? Sheet-fed? Tractor? Label? ESC/POS Thermal or Impact?

The printer is a Epson LX-300+II (Tractor and Impact)
But, reading your messages, i talked with my client, and he accept to use tickets, so, thank you for your help.