3gallon pots seem to be around the ideal size, bigger doesn't hurt. If you aren't going to be able to water them regularly then going bigger will help keep the plant/roots from drying out as badly during periods without rainfall/watering as well.
i would definitely get a 3 or 5 gal pot and keep it with it. transplanting puts a halt on growth and since lowriders and easyriders have a limited life cycle they will not reach their maximum potential.
5 gal is a bit of an over kill for an easyryder you will only be wasting nutes for like 2g extra, not that its wrong but 3gal is enough for that strain
3 gal should be good. Some larger plants do better in 5 gal but start with a three. Plant the seed into its final pot autos do not enjoy transplants. They can be done but it stresses the plant so if you must do it when its young and into the final container.