I found this useful guide to image resolution and sizes at creativebloq.com
It is a usefull guide to get your head around pixel dimensions, actual printed size and resolution before preparing artwork for print.
Print Image Resolution
When preparing your images for print we recommend an image resolution at 300ppi at print size for all work that may be viewed close up and only drop to 200ppi or 150 ppi for images that may be viewed from a distance of over 1m away such as posters, banners and roll ups.
Master Image Files
It is useful to always keep your original image with the maximum resolution as a master image and make copies that are smaller when required for low resolution or smaller image use by using ‘save as’ after manipulating and resizing the image. If you do this you will always have the original image to go back to when required. It is also worth bearing in mind that if you edit and re save a JPEG file as a new JPEG file you will loose quality each time you save it. Depending upon your JPEG compression settings the loss may be slight but good practice is to make all changes together and then ‘save as’ the new file.