Question How do I place two images side-by-side in a custom inspector, fitted to current width?

I'm trying to place two Textures side by side in a custom inspector. I'm currently placing them in a Horizontal layout like this:

Code (CSharp):

1. GUILayout.BeginHorizontal();
2. GUILayout.Box(myScript.tex1);
3. GUILayout.Box(myScript.tex2);
4. GUILayout.EndHorizontal();

However this renders the Texture really big, as shown in the attached image. If it matters the textures are 640x576 and 1920x1080, so I suspect it is trying to render them at the full resolution. How do I constrain this so that each texture takes up 50% of the current inspector width?