First, make sure you know the difference between image size and file size.
File size is the size you see in Windows Explorer or Mac Finder. This is the size of the "container" that holds your pixels.
Image size is calculated from the width and height dimensions of the pixels in your image. You can find this under Image>Image Size in Adobe Photoshop.
The following numbers all refer to Image size. These are the minimum recommended image sizes.
|Print Size||Image Size|
|8x10 or smaller||5-7 MB|
|30x40||no more than 100 MB|