There were four main types used on land: standards, banners, pennons, guidons and pencels. Ships also used streamers.
I will offer some definitions that are not inconsistent with medieval use, but since medieval definitions were not consistent, they will not agree with all medieval use.
Banners were rectangular or square. The others were triangular.
Standards and guidons had a split end. Pennons and pencels had a single end, pointed or rounded.
Standards were usually the largest flags and pencels the smallest, sized to be born on a lance without impeding its use as a weapon
One or more badges could be displayed on all of the above. Almost always, arms were only displayed on banners, pennons and pencels. The one exception that I know of included the arms of the Count of Nassau on an escutcheon among other emblems painted on a standard captured at Grandson in 1476.
14th century banners ranged from 60-90% of the height in breadth, with three feet high and two broad being a fairly typical proportion and size, judging from the iconography. They evolved to square proportions under the Tudors.
Six feet seems to be a good length for a pennon.