Modern phones take photos that print beautifully at A2 and even A1 - provided you give the file a little care before uploading.
Start with the highest-resolution version of the file. In iOS, that means sharing the original from Photos rather than a compressed message. Aim for 300 dpi at print size: for an A3 print (297×420 mm) that's roughly 3500×4900 px.
Crop to the print's aspect ratio before you upload. Our editor will show you the safe area, but planning the crop yourself avoids the family member who ends up half in shot.
For colour, trust your phone's default profile. Heavy filters rarely survive print - subtle contrast and warmth adjustments do.