What is the resolution in your Project Properties, and what is the scale of your preview pane? Those are the two things that control how large your video frames will appear in the Vegas preview pane. (Also, what is the resolution of your ultrasound video clip?)

So, to make your preview larger, first make sure that your preview window is set to Full size. Generally, you should set your Project Properties to match the source video you are editing. The resolution of your rendered video is controlled by the parameters you set for the rendering operation. So, if you want to increase the resolution of your final video, render it with the resolution you desire it to have.

Others here might not approve (I haven't actually tried this, either), but if you have low resolution video and want to increase its resolution and also run it through a video effect to sharpen it (for example), you could try setting your Project Properties to the resolution you want to render to, so you will see in the preview window how blurry your low resolution video will appear after it has been blown up to its ultimate size once it is eventually rendered. Then you should be able to play with the sharpen filter to hopefully get the effect you want. This is something you could try experimenting with, but, as I said, others here might shoot this idea down.