This is excerpted from the tip of the day compilation:
"...First Layer Settings (These are applied to where the Bed touches your model. If you have a raft, that means the First Layer Settings will apply to your raft).
First Layer Height: This % will take a % of your Primary Layer Height. If you are printing with a small Primary Layer Height like .1 mm I would recommend a First Layer Height of 250%, to get about a resulting .25 mm first layer height
If your First Layer Height is below 100%, the extrusion amount will remain the same, only the Z-will change, but if you increase above 100%, the extrusion amount will scale accordingly.
First Layer Width: the extrusion width of your first layer, you may find that your first layer sticks better with a thicker extrusion (100%+). I don't have too much experience with this, but I think 125% or 150% would be good starting points.
From reading the above if you want a wider first layer you could set your first layer height to below 100%. Since the extrusion amount will be the same, and the z height reduced, you would end up with wider extrusions and more overlap since this setting will not alter path.