Hi tifus, I think what you mean is how to tell a matrix (impact) printer to print x number of characters per inch (cpi) unfortunatelly configuring matrix printers takes time and patience. They have some limitations in fonts and you'd have to settle for whatever type of fonts they can print. You may want to check IBM webpage for filters. I don't think this is a limitation in Linux. Have you got a chance to test this with a different printer to see what kind of results you get?
good luck!