Hi holm,
you CAN resize the send knob - but not larger than the sendlist height though. The default one doesn't use the available space completely and you can make a knob stack that uses that space.
It is a rather simple knob stack which can be made with
knobman. You can download knobs from the
knob gallery and modify it to your needs.
For the Standard track layout make it 12x12px, 14x14 for large and 10x10 for small. The original one has 15 frames, it's enough for that size, it is a little smoother with more frames. I have tried it with "standard" and "large", so "small" should work too.
The color of the bar you show in your screenshot is the "mcp_sendlist_meter.png". The upper part is the resizing "value-part", the lower part is the background. In the default "mcp_sendlist_meter.png" the lower part is empty, but you can add a color there. It's very transparent and therefore almost unvisible, you need to make a dark layer in PS or Gimp behind it to see it.
You sadly CAN NOT change the height of these particular send bars, same with the zeroline.
The "font" and "margin" affect only the text, that's right.