I am no expert on primers. However I did find a professor at Penn State University who may be able to assist. Just follow my blue underlined hyperlink to his webpage on "Polymerase Chain Reaction Protocols. (His email address is on that page)
He refers to picking primers as you have inquired (see below)
"Using the Primer Selection Program...
...Keep in mind that we prefer to pick primers between 17-21 bp which are found in the last half of the cDNA and result in a product of 100-400 bp."
you choose your primers differently depending on what you want to do
you usually use a oligomer around 20 nucleotides long because of it reduces the likelihood that the primer will bind to a random sequence and because you need to maintain certain temperatures for proper annealing and elongation during PCR. If your sequence is too short or too AT rich, then the primer will more likely be able to be denatured from your DNA during annealing and this causes a loss of specificity for your DNA sequence.