Thank you for visiting the HP Forums! A great place where you can find solutions for your issues with help from the community! I came across your post and wanted to assist you. I understand you are having picture quality and need assistance with size the picture using your HP Sprocket. Don't worry as I have a few suggestions which should help you resolve this issue.
I did go through the specifications, please find the paper and file size supported by HP Sprocket.
The printer supports the following paper and file sizes:
7.6 x 5 cm (3 x 2 in) HP ZINK® Sticky-Backed Photo Paper
BMP, GIF, JPEG, PNG, and TIFF (iOS only) file types
To improve the quality of the print:
Clean the output rollers with the blue HP ZINK® Smartsheet from the same pack of photo paper currently in the printer. Load the blue HP ZINK® Smartsheet into the input tray with the bar code facing down and the photo paper with the print side facing up, and then send a print job.
The HP ZINK® Smartsheet calibrates the printer to provide the best print quality for printing with the specific pack of HP ZINK® Sticky-Backed Photo Paper. The HP ZINK® Smartsheet cleans the paper path rollers during calibration or when you need to improve print quality.
Load photo paper with the Smartsheet to avoid an Out of Paper error message.
Make sure images have a minimum resolution of 818 x 1258 pixels per inch (ppi) for optimal print quality.
Allow the printer a few minutes to cool down between print jobs.
If the issue still persists after trying out the steps try updating the printer firmware and check if it helps.
Update the printer firmware. On your mobile device, make sure it is connected to the Internet, and then open the HP Sprocket app. If there is a firmware update available, a firmware update screen displays. Follow the on-screen prompts to complete the update.
Also, refer the printer user guide to know more information.