I am finding a lot of 3D blocks on here that I would like to use, but they are either really large or really small when I copy and paste them. Is there a way to scale them to the right size? My drawing is 1/4".

Hello reverheart, 2D or 3D blocks it's the same thing.
You are suposed to work at scale 1/1 and only when printing you "scale" the print ( but not the file ) with the dialog box of your printer ( but it's an other subject).
If you want to scale them to the right size you must verify in what unit they were drawn.
Type UNITS and take a look if there is an "insertion scale unit" and what it is.
If you set your drawing with the unit that you use ( inches , feet , millimeters , meters , etc ... ) then an insertion ( and not a copy/paste ) will bring the file ( that will become a block inside your file ) to the right scale.
If you use blocks of unknown persons then do a good check before using them.
Ask for more and have a good day. 
 
Take a look at this :
http://www.cadtutor.net/tutorials/autocad/units-and-scales.phpphilippe JOSEPH2013-08-06 12:53:07

slt a toit championle problem que tu pose est un problem récurent pour les débutants dans autocad, il faux ce dire que tout dessin est fait a une échelle exacte. dans les version françaises d'autocad en majorité tu aura m cm mm qui sont des unités régulièrement utilisées dont verifit juste avec quel unite ((j'insiste sur le mot unite)) ton dessin de basse est et l’unité du  block que tu souhaite insérer la tu pourra choisir la quelle des unités tu souhaite conservées. .........bonne comprenssion mon grand .......