Wide fan, narrow fan, thin base, flat base! There is no correct answer, they are all right and completely down to your personal preference but let’s break it down!
A wide fan will make your work look fluffy, light, and soft and can also make the length of the lashes appear shorter as they are not so dense in appearance. When creating a wide fan be careful not to make the stem to short as this will mean a small attachment to the natural lash and compromise your client's retention.
A narrow fan will make your work look thicker, denser, and more uniformed and is perfect for creating that spotless top line. When creating a narrow fan be careful not to make the stem to long as this will close your fan and end up making your work look messy and not uniformed at all.
A thin base will make your work look tidy, you can attach a thin based fan on top of the natural lash, to the side and from the bottom and so is flexible when it comes to attachment, a flat-based fan will create a darker impression along the lash line but can only be placed on top of and underneath the natural lash.