The best way to fix a muddy area is to put down large clean crushed rock and then top that with something more suitable for your cause such as driving, walking, pushing a wheel barrow, etc.
2 1/2" Clean Crushed
To top this larger crushed rock you may use a minus rock, which is popular for driveways. It has fines in it that get between the bigger rock and help it to pack down. You may also use a smaller clean crushed rock which will not pack as well but tends to look nicer. Take a look below for some popular choices for your application. Plan on putting about 3" deep of the top-course rock.
Extra Parking Area: You may use a clean or a minus. A clean rock will look nicer and will drain but may shift out from under the tires and create ruts in the gravel. A minus rock will compact in tightly and create a nice driving surface but may not look as nice and will not drain. You may want to consider the amount of traffic in the area and weigh looks versus function.
Got Mud?Whether it's a walkway, pathway or driveway area that has acquired too much water or created a muddy spot, there is one common solution. When mud claims an area on your property the first step is to assess the damage, decide how deep the mud goes before it hits solid ground again.
2 1/2" Clean CrushedTo determine what size of clean crushed rock you need you will have to assess how deep your mud actually is. You need to pick a rock size that will sink into the mud, hit solid ground and then begin to stack up on top of each other until you have a solid surface of large crushed rock that has buried the mud. For example if you have 4" of mud and you use 3/4" clean rock, it will sink into the mud and disappear. It would take an atrocious amount of rock for it to reach the top of the mud, if it ever does. For mud 4" deep, I would suggest a 1 1/2" - 2 1/2" Clean crushed rock. It is big enough that it will stack up and rise above the mud.