Below is confirmed working in CBs CSS override functionality.
Code:
.cb_template .cbActivity .cbMoreLess:not(.cbMoreLessOpened) .cbMoreLessContent {
max-height: 200px !important;
}
The initial values are set within the JS. There is a height tolerance of 400px you won't be able to override. This means you need to keep in mind 400px will be added to whatever value you set. So if you set the limit to 200px that means the maximum allowed is 600px. The default height limit is 400px this means at 800px it activates the Read More and shortens it to 400px.
Please keep in mind while CB Activity does provide a module it is not designed to be crammed in to left/right module positions as it's just too much content to be put there like you're currently doing.