I am told that in order for photos to be suitable for printing purposes I need to export the digital files at 300 pixels per inch or more. However, in the latest version of Lightroom 4.0 (not CC) there is no way to customize pixels per inch. You can simply export as "small", "large", or "maximum available." You can adjust the "long edge" but there is nowhere that allows you to fix PPI. I just want to make sure that if I'm exporting as "maximum available" it is exactly 300ppi. It seems odd that it won't tell me what the resoution is under that option. Any help is appreciated.
PPI literally has no meaning in a digital image. I don't care what any lab says. What matters is whether or not you have enough pixels in the image to print the size image you want. If it's a 4 x 6" print, and you think you need it at 300 PPI then the math would look like this:
4 inches X 300 PPI = 1200
6 inches X 300 PPI = 1800
So to have a 4 x 6" print at 300 PPI the pixel size would need to be 1200 x 1800 pixels. The PPI setting in the file could be 72, or 300, or 600, or whatever. It wouldn't change the quality of the image at all because the pixel count is what determines the quality of the print. If you need a larger image all you need to do is the math using that same formula to determine how big the image needs to be at a minimum to produce that quality of an image.
The digital image doesn't really have a PPI setting. The quality of the digital image is simply governed by the number of pixels it contains, and that's all. Placing a number in that PPI setting only serves a purpose to notate the file for you or for someone else. It doesn't affect the quality of the image in any way whatsoever.