I use Versacheck to print my checks.  I was told years ago many check scanners were Magnetic InCoded Readers (MICR) and had to read the numbers with  the help of the magnetic ink.  Now units are optical readers.  I usually use print business size checks through my accounting program.  I buy blank stock off of ebay or cheapest place i can find.  You can print smaller checks with these programs but I like the bigger checks.  Still have a bunch of smaller checks that  I bought years ago for taking with me and still running on that supply, those wre from a mail order place that I saw an ad for in the insert in a Sunday newspaper.