It will be a tight squeeze. Note that the rooms in all the "value" resorts are 260 sq ft with two double beds. If you can possibly upgrade to a moderate you will have 314 sq ft (except Caribbean which are 340 sf). The extra space helps.
It is generally to your advantage to rent from off-site. Main reason is the
ECV will always be available. If you change parks in midday there is a strong chance you will not be able to get an ECV, as they will only rent out those that have a full charge ojn them. Also, you will be able to use it at Downtown Disney and, if necessary, the water parks, where normally only wheelchairs can be rented.
Also the
ECVs from off-site are better in that they have three wheels instead of four and are much easier to maneuver, and they have much better control of speed (maximum 7mph compared with Disney's about 3).