Ye Olde Inn

Trying to print my own cards online using custom card websites but they seem to over or undersize the cards printed and complain about resolution. I've used the various Hero Quest card making apps online do these apps not do it properly then? I have tried resizing and playing about with it but I cant get them to fit properly even using really large or mini playing cards. Thought someone here might have some info or tips. I want to print a whole load of new cards. I don't really want to be buying ink and printing myself when I can happily pay someone and get a nice finish.