How do I set file size of iPhone pictures

Unfortunately you can't. Below are the camera specs for your phone. There are no settings; you would have to get a good digital camera to do so.

8 MP, f/2.2, 29mm, phase detection autofocus

1/3" sensor size, 1.5 µm pixel size, touch focus, geo-tagging, face/smile detection, HDR (photo/panorama)

Hi 🙂

The pixel density PPI doesn't reflect the quality of a photo, but the quality of a printing instead. A Retina display looks so nice because it has a high pixel density. Pixel density only matters when it comes to printing. Let's think about printing your 8MP photo on an A4 paper sheet and on an ad panel of 6x4 meters. With the same amount of pixels, printing on the A4 paper sheet gives you a better look since the pixel density is high; on the other side, you have to scatter 8MP on the area of 6x4 meters which the pixel density is surely low, and the look is not the same anymore.

And also, size of a photo file doesn't always reflect its quality. You can always have a 50 MB photo which quality is crap 🙂.

What iPhone can provide you are good looking photos right "out of the box", but not the "top-quality" photos.

Just take your pictures, import them to your computer, and export them at whatever dpi you want the output image file to be. An iPhone 6 with its 8MP camera is never going to produce a photo file larger than 2.5Mb - 3Mb - it simply does not collect enough image data to ever produce such a large file size. Even the 6s with its 12MP camera does not generate photo files anything close to 7Mb. The original image of any iPhone or iPad is as high a quality as it will ever be - there is no way to change that native image file captured by the camera sensor.

dots per inch refers to the pixel density of a printed image, not the actual camera image file. Your iphone 6 images are 3264 x 2448 pixels, regardless of what dpi they end up being printed at. You cannot change the native resolution of the camera image file. If you print your iPhone 6 image files as 8"x10" images, they would be at 306dpi. Or, conversely, if someone has asked you for a camera file to be used for printing images as 8"x10" at 300dpi, you would need a minimum of a 7.2MP camera.

(so did they request a 7Megabyte image file, or did they say they wanted a minimum 7Megapixel image?).

I have Photoshop on my computers and Photoshop Express on my iPhone. Keep in mind that Photoshop cannot increase the resolution of the photos taken with the iPhone. You can edit the photos, correct defects, tweak the exposure, etc. but you cannot add "additional pixels". After editing PS sometimes increases the file size but that is not due to better resolution. PS is basically used to edit photos, that is all.

And BTW, the camera on the iPhone 7 is far better than the camera on the 6, something to consider if you want to stick with iPhones. Among the improvements are a faster lens and a 12 megapixel sensor. However, it is still no match for a good mirrorless or DSLR camera.

Pixels are only part of the story. Of more importance is the sensor size and iPhones have a very small sensor. These articles may help.

http://newatlas.com/camera-sensor-size-guide/26684/

https://www.digitaltrends.com/photography/image-sensor-size-matters/

You are welcome.