This is kind of tricky because the SplitContainer isn't just one control. When you add it to the form, it creates two panels, too.
The trick here is that Panel1MinSize and Panel2MinSize are properties of the SplitContainer not the panels. If you click on the SplitContainer and not a panel, you should see them in the Properties window.
Clicking on the SplitContainer can be hard. You can either click on the bar between the two panels or click off to the side and drag until you overlap the control. Or use the dropdown at the top of the Properties window to select the SplitContainer.
Let me know if you have other questions!