When the biomorph gets too big for its canvas, I automatically reduce its branching depth until it's not too large. If you keep clicking on children that make it evolve towards larger entities, eventually it will shrink and look something like the biomorph in your screenshot.
Someone suggested that rather than automatically reduce its size, I should just make them unclickable (ie, you can only choose a child that fits within the canvas).
What do you think? Any other ideas?
Edit: I just updated the implementation so that instead of automatically reducing the child biomorph's size if it's too large, it simply becomes unclickable (you'll also get an notice if you try to click it). Should be a bit better -- thanks for the nudge!
https://i.imgur.com/aI0zDw9.png