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 &quot;Danger Zone&quot;. 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.