If you select the color matte on your timeline and click the Applied Effects button, you can open the Motion properties and uncheck the Constrain Proportions box so that you can scale the matte any way you want.
You can also use PrE's Titler to create a colored rectangular Shape (along with other Shapes), that can be sized, as needed.
What exactly are you using the Color Matte for - it appears that you want a colored rectangle to appear within your Frame, sized as needed?
You can also use the CROP effect to cut the size to whatever you like.
Interesting how we all have different ways to do something similar!
I wish I could answer your color question though, I would like to be able to do that as well.
Something for you to consider for your Color Matte color matched with a color that is part of another image on the Timeline.
Given you have a photo on the Timeline. The photo shows a red rose whose red color you want to capture in a Color Matte from Project Assets/View/New Items/Color Matte.
Photo on Video 1. Side by side the Color Matte (any color).
Select the Color Matte clip and, from fX Effects, drag the effect Image Control/ “Color Replace” into the Color Matte clip.
Then, with the Color Matte clip selected go to Applied Effects Tab/Applied Effects Palette/Color Replace Panel expanded. (Keep away from the Setup icon on the Color Replace Panel.
Keep the Color Matte clip highlighted, and move the Timeline Indicator to the left to be on the photo clip which is NOT SELECTED.
Important: At this point, the photo is not selected, but the Timeline Indicator is positioned on it so that the photo is displayed in the monitor. The Color Matte is selected so that the Color Replace Panel expanded is visible to the right.
With the mouse cursor, left click once on JUST the eyedrapper for the Replace Color option in the expanded portion of the Color Replace Panel. Move the mouse cursor (which presents as an eyedropper) over to the red rose on Edit Mode Monitor screen. Click on the red rose.
Take a look at the resulting color for the Color Matte. How close did you come for a color match?
After that you can use the Color Replace Panel's Setup icon to fine tune with a check mark next to Solid Colors and adjusts from the Sensitivity Slider.