Never had a cow in the pen. 100% agree on making it big enough or you just wasted your time and money. Dad built one too tight and deer refuse to jump it. That pen is now opened. We build them circular with rebar. On rock we drill into it, with soil we get longer rebar. Unfortunately it doesnt solve coon problems, but my 3 sons will have some fun with them after deer season.
Out of 10 or so feeders, we left 2 without pens to still hunt hogs. Those boogers are just too destructive. Auodad (including very young ones) and dam goats have gotten into pen, but never a cow on our place.