Skype is displayed in Default View, with everything in a single window. To resize the window:

• Click on a corner of the window and drag to make the window bigger or smaller.
• Drag the sidebar on the right of your Contacts list to widen or narrow it.

You can also switch to Compact View, to view your conversations in separate windows. Click View > Compact View, or click the view button  in the top right corner of Skype.

Each window can then be scaled and moved around your screen, so you can organize them in the way that works best for you.

To revert to Default View, click View > Default View, or click on the view button again.