The rule with airlines is the carrier has to fit under the seat..so it depends on what plane you are on. Go to Delta's website and seach for animal or pet travel it will tell you. You also will probably have to have a health certificate with you also and it can only be done 10 days before travel..again their website should tell you.
In regards to going the bathroom just take a peepad into the bathroom with you but don't let the attendent see you take her into the bathroom. Have a big purse you can sneak her into. You are not suppose to take them out of the carrier...
If you do a search on YT for airline travel you will see many threads about this.
Good Luck and have fun.