The UA 787 BF seats have rather narrow footwells, except for the bulkhead rows (1 and 4). More height than under a regular recliner F seat on other plane types, yes. If you flew CO prior to the merger - I think the seats are the same as the business class seats in pmCO 777s.

I haven't flown UA with a dog since the merger (and their change to "dogs must fly cargo"), so can't comment on their current acceptance of the larger-than regulation size carrier. Just be aware that the XL Sturdibag is 2" longer, 5" higher and one inch wider than UAs pet carrier regulations allow. (20*16*12 compared to 18*11*11). However, it's squishable and as long as our dog won't fill it out completely, I don't expect there to be any issues.
Remember that there's not really a fallback option should the pet be refused - UA does not allow pets to be checked as baggage, and their petsafe program isn't something you can arrange on the fly while at the check-in counter for a flight departing in two hours...