Custom Papersize on matrix printer

I program to print a cheque book.. i set printdocument papersize to 3 inch height and 8 inch width with new papersize constructor.. it works finly in printpreview dialog but when i try to print it, it does not print correctly as preview control.. it print one cheque book and throw whole single page as printer default papersize in set as A4.. how can i set printer default papersize to custom.. i am using vb.net.. thanks in advance for your support.