An answer to your question and some advice from a UX point of view:
Popups can be a tricky subject indeed. Usability wise a popup should almost always be used for a very limited amount of data / information and possible actions. If that is not the case than it's probably better to design and implement a normal application page. Although every project is unique - meaning there are no universal UX answers or rule of thumbs that always work - you could start working with a max of e.g. 4 dataview fields and 2 actions (excluding the close and cancel) for popup usages. If you have something bigger than that, use something other than a popup or come up with a really good reason why it should still be a popup.
With regards to your sizing question. Using the css '!important' statement you can override the automatically calculated sizing values that result in inline styling. (so override the height and width property) Be mindful to properly test it on a lot of screen sizes though as standard behaviour no longer applies.