The Profile Picture here refers to the business and not your personal picture.
The minimum size for Profile Picture is 180 x 180 pixels but in these days of high resolution devices, it’s best to have a minimum size of 360 x 360 pixels.
You can upload higher resolution images, but always ensure a square (1:1) aspect ratio. For example, your image can be 800 x 800 pixels or even 1,400 x 1,400 pixels as Facebook will compress the image upon upload.
Do note that Facebook will apply a circular crop on your Profile Photo, so you need to make sure your image is not cropped at the wrong areas. This feature is rolling out currently, so you might still see some Business Pages with a square Profile Photo.
Some examples of what the top brands are doing for their Profile Photo. Simple and clean works best and don't forget that circular 'crop'!