I have read what everyone else has said and it seems like all of you are from the USA.
I am an Australian but have been to the USA once on a holiday together WITH A FRIEND WHO USES A POWERCHAIR. We have seen the set up for public toilets there, so I know what you are talking about.
My answer to you is YES, it is appropriate to use the larger stall. You weren't to know that there was a person coming in after you. If all the other cubicles are full - then you are within your right to use it since it is there to be used and it is NOT designated to be used by people with impaired mobility.
In Australia, things are different. Most public toilets and the ones in the shopping malls have larger toilet SEPARATE to the others. There are many advantage to this. The toilet is unisex and therefore if the person in the wheelchair is a male but the caregiver is female - BOTH can go into the same room especially if the person needs total assistance. Because it is designated, (usually) there is no queue to wait for. Mothers with prams can also use it. This is better than causing congestion in the regular toilets (either with a pram or a wheelchair)
Because it is a designated toilet only for people with a disability or parents with children, ordinary people who use these toilets are looked upon in the same light as people who parked in Mobiliy access parking zone (and YES!, there are people who DO use them purely because they are inconsiderate and give no thought that my friend and I are IMPATIENTLY waiting outside!)
Hope this makes sense to you Americans.
It is probably hard for you all to read my Aussie typing accent. LOL