Smartphone Wallpaper Dimensions

Hi there, so ive searched the webs for the past 3 days and cant find a defenite answer to my question, i have also searched here on the forum and i cant seam to find it ither. If for some reason there exist a reply or a topic related please refer me to it and feel free to close this post.

Any way ... my question is this... My mother Bday is geting close and shes been nagging my head for a costum smartphone wallpaper, but the thing is the smartphone she has is new brand that came to the country, so i kinda spend the past week searching and learning with no luck cant find the answe anywhere, even the shop clerks dont have a clue to many stuff and keep saying "Im sory this a brand that is new on the country and even we dont know much about it including what your asking".

So if i may ask here as a last resort cuz i also asked on facebook pages and twitters and i get no reply, can someone tell me the dimentions a photoshop project needs to have in order for me to desing a good quality wallpaper for the smartphone ?? Also i almost forgot ... her smartphone is a Oppo A15 in case this helps for some reason.

Regards ASilva