Storing Cooked Food Hot

First of all there's portion size: that should not matter. I'm assuming here that you transfer the food from cooking straight into the oven to keep warm, and not re-heating after cold storage. This means that your food will integrally cool down to the ambient temperature in your oven.

For temperatures there are lots of resources available, such as Fact sheet or Governmental guidelines. The first link provides the only real reference to timing I could find. It seems like from a food safety point of view there is no clearly defined maximum storage period, as long as you keep the food out of the "Danger Zone". Regular stirring of the food and the right temperature apparently keep it safe for long enough for the authors of these resources to think it not worth mentioning.

There is how ever always taste and texture to consider. I would not recommend keeping any dish warm for many hours, regardless of the temperature.

There is a related question here, which was more specific in only covering broth/stock at high temperatures, held indefinitely.

As stated in the question (and in my answer to the linked question, with another source), the usual cutoff is around 60C/140F. Just keep food above that temperature. Very few bacteria (or other dangerous microorganisms) can survive above that temperature for long, and if they are present, they will die off.

(That said, please monitor and verify that the food always stays above that temperature. Bacterial spores can survive above that temperature, and if you start dipping below 60C/140F for significant periods, you could grow nasty things. This is part of the reason why many government inspectors are so concerned about buffets -- something that's quite warm, but not steaming hot, is potentially a worse food safety hazard than something like leaving food sit out at room temperature, due to the elevated bacterial growth that often happens above room temperature.)

As noted in some of the comments, the practical limit is until the quality of the food degrades. High temperatures will tend to dry food out (if not sealed completely) and will gradually degrade some flavor components. Textures will also gradually break down, which make also make the food less desirable. (Just about anything that has any moisture will eventually become soft and mushy.)

Portion size is probably irrelevant if two things are true:

• the food is above the given temperature when put into warm storage
• the ambient temperature in that storage is kept above that temperature

Heat will eventually equalize - and has nowhere colder to "go" in such a setup.