Formitize is optimised to be used on both Apple and Android devices.
Device size is up to you as a user / business. Many clients like using an iPad mini or Samsung Galaxy tab as they are small enough to carry around but big enough to be able to easily use especially when completing forms. You will need to keep an eye on any updates that may be required over time.
Note: When purchasing a Samsung device, ensure that it has access to the Full Google Play store as some features in Formitize are not available in the limited Google Play Store.