Healthy cannas for sale rhizomes/plants are supplied bare rooted with excess leaves and roots trimmed. Example pics of stock dispatched.
Size of divisions do vary, some cultivars will have big and chunky rhizomes with new shoots spread apart whilst others will be small and tight with new shoots tightly packed together however all will have a minimum of (2-3 new eye growths/stem growths on rhizome).
I have trialed cannas for postage conditions many many years ago (packed and sealed as for dispatch and they bounced around in the boot of my car for 2-3 weeks), once planted out they bounced away in new growth.
Canna rhizomes are a plant that will last very well if there are delays in the postage system however overwatering small shoots with no rhizome can cause plants to rot.
Upon receiving your plants place in a shaded area if unable to plant out for a few days, large pots are great however garden beds are better, turnover soil planting position and incorporate some fertilizer or animal manure, plant it, water it, mulch if in a hot summer then wait for plant to resettle and send out new growth before increasing water. In winter with regular rainfall at my new location I simply don't water newly planted rhizomes.