All options for custom listings can be found here.
Suggestions for sizing are as follows:
2x4: best for one to five word phrases and quips
4x6: best for two to fifteen word phrases, quotes, and quips
5.5x6: best for ten to forty word phrases, quotes, quips, or a poem if ten or fewer lines
6x8: best for thirty to one hundred fifty word quotes, poems, and vows (or anything shorter, if you'd like more white space)
Any questions can be sent to email@example.com.