There is not an easy answer to this question because everyone’s iPad setup is slightly different. So your iPad may have more memory available to it because it has a faster processor or fewer apps running in the background than another person’s setup. For this reason, we recommend images be sized to 72dpi and around between 1000-2000 pixels on the short side for bug free performance. FULL res images will tend to crash on older iPads so this is definitely not recommended.
The good news is that if you are using Shoot & Sell with ProofShare with our recommended workflow you do NOT need to resize your images at all! You can import high resolution images into ProofShare which will automatically downsize them for you to the optimal size for your iPad type.
After your client chooses their favorite images in ProofShare, you can choose to save JUST those favorites to your iPad (or Dropbox if you have WiFi) and those images will be at a size that works perfectly for Shoot & Sell!