If the web page happens to be one that's displaying an email then it's important that you DON'T just use Ctrl+P (or File > Print) to print it out. You need to look for a 'Print' option within the actual page itself; that will just print out the email without the surrounding space.
For all other web pages you're bound to end up with very small print if you make something which is perhaps about 70cm wide on your screen fit onto the 21cm width on an A4 sheet in portrait format. However the reduction won't be as great if you print in landscape format. (When you hit Ctrl+P, or go to File > Print, don't just continue directly from there. Choose 'Properties' first and tell your printer to use landscape mode).
Otherwise the only way you can adjust the print size is to copy the text from the web page and paste it into a Word (or similar) document. You can then make the text any size you want.