Use this:
.tc-tiddler-body { max-height: 50vh; overflow-y:auto; }
You can, of course, adjust the max-height value to suit your specific needs. You could also replace "max-height" with "height" which would force all tiddlers to be the same height, even if their content is shorter.
note: "vh" units refer to the current height of your browser window.
If you resize the window, the tiddler heights will adjust automatically.
If you use other kinds of CSS units (e.g., "em", "px", "in", "cm", etc.)
then the tiddlers heights will no longer be adaptive to the window height.
enjoy,
-e