When you upload the photo to Instagram, it will save high-resolution versions of your photos to your phone. If you want to save filtered photos to your phone, then you have to turn ON the "Save Original Photos" setting which is located in your "Account Settings". Instagram saves photos in the highest resolution size possible. The maximum resolution on both Android and iOS device is 2048x2048, but your phone's resolution capabilities may be lower than that. The photos imported from iOS device's camera and uploaded to Instagram may save at a much lower resolution.
If you are using Android phone, then the photo resolution capabilities are different for each phone.
Apple iOS Device:
If you are using iOS device and taking photos with the Instagram camera, then the photo resolution is :