There are dimension limits (they are posted in the disABILITIES FAQs thread bus information).
There is also information in there about actually driving on. For the lifts, it is recommended to
back on, which puts the weight of the driver and
ECV closer to the bus, rather than out on the unsupported floor of the lift.
For the buses with ramps, it doesn't matter which way you go on, but if you have a large ECV it may be easier to manouver into the bus spot if you back up.
There are now signs on the buses recommending ECV/scooter users transfer to a seat rather than ride the ECV during the trip. That's because ECVs are not designed to be fastened down and most of the spots where the tiedown straps can be fastened are close to the bottom. That means there is a possibly it could tip over when the bus takes a corner. There are seats right across from the wheelchair spaces that are labeled as needing to be given up if needed for someone with a disability. If the person using the ECV is not riding on it, the bus driver will usually be able to fasten the ECV more securely, higher up over the seat.
Wheelchairs are different since there are many higher places that straps can be fastened.