There is no hard and fast golden rule. Typically, you want to make the page load with as few HTTP requests as possible. However, Ultimate's goal is to make the page load as fast as possible. While reducing the number of queries is key, this is not the only factor. For example, if you have a page with one 100k js file, no css, no images, no other requests. This page will load faster in most modern browsers if 4 25 thousand Js were requested on the page because 4 requests can be parallelized over 4 connections. However, your typical web page has from 30 to 100 requests, and they will load much faster, combining files, since most browsers are limited to 6 connections per host, and there is latency when opening new connections, as well as problems with blocking with javascript and css depending on where no other connections will be started until js or css is loaded. It may also depend on whether these resources are in the head or not.
Thus, it all depends on your page and the target browsers. Personally, I use 50k as the maximum size for a single css or js. This, admittedly, is not extremely scientific. Its a nice round number and the one I find holds a few typical css or js files and is not so large that it is exorbitant.