Well, that model number does confirm an Alpha.
For larger pictures I resize in Elements keeping size the same but reducing pixels per inch.
Sending or posting from a phone often reduces size for easy emailing. There should be settings to keep them full size in your phone.
With new 5 x 8 Alphas with a 5HP spindle hitting over 36K I would say you are offering at a good price and wish you a quick sale.