I've always come across friends who complain that their fingers are "fat". One of the trick to making your fingers look slimmer is to make your nails longer giving an illusion of longer fingers. However the growing process may take a long time and for some, their nails are so vulnerable that they will never reach the desired length at the same time for all 10 nails.
Colours can make wonder for this purpose. Try to use nail polish colours that are very similar to your natural skin tone. By blurring the line between your nails and finger, it produce an optic illusion that your fingers are longer. Other options are pastel pink and beige undertone or simply use a clear polish. A rounded fingernail with clear polish will give you the longest nail look.
A French manicure gives the look of all-natural colour, but add an even and seamless white line on all nails. This makes your nails appear healthier and longer. However, be sure to keep the white portion at reasonable thickness making it as natural as possible.
Another way is to camouflage the area that you do not like. Patterns on your nails are effective to visually slenderize your fingers. Try to use strips from the nail bed to the tip with any colour combination but it is super effective with strips in nude colour combo.