Barbara, if Steve's solution does not make it large enough, you are not limited to s, m, and l for size - you can enter a number of pixels instead.
A pixel is the unit of length that can be specifically identified on a screen. For example, a typical screen resolution might be 1920 x 1080 - that means that it is 1920 pixels wide by 1080 pixels high. I believe the "l" setting for images makes it 600 pixels wide, but you can make it more by making the size line:
|size=700 (or any other number you want)
Be careful, though - you'll need to look at it on the view page after making these changes to be sure that it still fits in the width of the profile display.
You might also leave the size alone, but change the caption to:
|caption=Gonaby Breece, heir of Frederick D. Breece<br />(click [https://www.wikitree.com/...etc
. HERE] for full size)
where the https://www.wikitree.com/...etc
. is what appears in the address window of the browser when you are on the full size image page - just copy it from there and paste it into the editing space.