what file size is an SE's photo file?

2-3mb is correct file size for 12MP images taken on an iPhone SE (and most later iPhones). If using iOS 11, you can change from HIEC (High efficiency format) to jpeg in Settings->Camera->Format and it will produce larger files, but retain the same high resolution.

iPhone's do not and have never taken RAW images. That's a DSLR camera format only.

File size however does not really impact image quality. a 2-3mb file from an iPhone is still a high quality image at 4032 x 3024 pixels.

Details here:

iOS Camera Resolutions - Apple Developer

Also read here:

Cameras with very different files sizes at 12MP: Beginners Questions Forum: Digital Photography Review

You can shoot in RAW on an iPhone with iOS 10 or newer, but not with the native camera.app. But there are a few third-party camera.apps that support shooting in RAW format. I have not tried any of them, however:

How to shoot RAW photos on your iPhone or iPad | iMore