It is widely accepted that the right hand fingernails be used Longest to pluck the strings as this will give a greater control over the volume and tone of the notes you play. It is therefore assumed that you use nails for the duration of this method.
If, however, for one reason or another nails cannot be grown or kept, you will have to focus attention on the fingertips in place of the nails. Nail length and shape is mainly dependant on personal preference but the most common shape is that which follows the curve of the finger-tip. Experimentation and listening for the best tone over a long period is advised.
Remember, the nails are to the guitarist what the stylus is to a record player and must be looked after accordingly. They should be filed and shaped regularly, working from the underside of the nail (Use very fine 'wet or dry' sandpaper).