Most Greyhound bitches are not very tall, usually only about the same size as a collie. You can also get some dogs that are small as well. And as for being cute - they are beautiful! So kind and gentle that you will be smitten.
Don't worry about leaving it for a few hours, as someone else said, it will sleep. Give it a nice cosy duvet type bed so it can snuggle into it - they do curl up but sometimes like to sprawl out full length. Main thing to watch is leaving food about - because they have only ever had access to food that is rightfully theirs, they consider anything left out is for them! This includes things like the butter or Sunday joint left on the table to cool. You'll soon learn.
Also one more thing, don't over exercise in this hot weather. Ashort walk early moring and late evening, and a couple of 'comfort breaks' during the day (in the garden is fine). Aw, now I'm getting broody!