I seem to remember the following limitations for DP. 25 rows of 40 "items". Where, an "item" is not necessarily one video. It can be a series of videos. There's no limit on the physical size of the feed file. The Roku partner site details these limitations.
FREE Windows desktop software for converting Direct Publisher channels to SceneGraph (SDK), for creating
BIF (Trick Play) files,
Roku (MRSS, JSON) feed files, and
FireTV feed files @
GitHub/rrirower.