I'm taking pictures of my little niece and I have a d3100 nikon?

But i'm not sure if I should set my image size to med like 3696x2448 which I have already been using for other photo's… And if I leave it on this size will i be able to print out 4x6 as well as large photo? With out issues of resolution problems? Can I just resize it in photo shop to a 4x6 if it is a large file?

Leave it on high resolution for best quality and use print adware to make it fit 4x6

To print 4X6 pictures, your camera should be capable of at least 1MP of resolution. D3100 Nikon has more than that and therefore, you will be able to print it in high quality. The setting you have determines the file size and of course the resolution. But… Will not cause much harm to your final intended prints of 4X6. If you are thinking of enlarging them at one point, larger is better. By the way are printing at home or at Professional photography printers… If outside, absolutely no concerns. If printing at home, make sure that the "printer settings" are proper to print " the whole frame" of your picture in 4X6. Because you have taken in a higher resolution, i general the prints tend to be very large unless you select the printer setting properly

Memory cards are cheap enough. Buy an extra or two and shoot on the highest resolution, at least large fine jpeg if not RAW. You can always make the dimension smaller, but you can't make it bigger. Suppose the parents want a poster?

If you use the highest resolution and basic image quality(4500x3000 or something is the resolution) then you will get pics that take up less space than high quality on the resolution you are using now.
Also, the default aspect ratio that the D3100(and most other DSLR's) take photos in match perfectly to a 4x6 print.