I hate my engagement ring. Can I tell my fiancé?
You must absolutely tell him how you feel! And together you can pick out an engagement ring of equal or lesser value. Most guys will understand that your opinion counts for a lot, since YOU are the one who will be wearing the ring—and literally wearing it every day! And guys are cautioned all the time to discuss ring styles with their partner before making a purchase, so you have good reason to speak up. If you think his feelings will be really hurt or if you think voicing your thoughts will make him angry and resentful (and take away from the joy you are both feeling upon getting engaged) wait a year before saying anything.
In general, a new ring is in order if you dislike the color of metal, you dislike the shape or cut of the diamond (round vs. square, emerald-cut vs. cushion, etc.), the ring is too modern/too glitzy/too old-fashioned-looking/too plain, and your style is just the opposite.
However, it is NOT okay to say you want a different ring because you think the stone is too small or of poor quality. Because that's like saying, "You didn't spend enough money." You would never want him to feel that his choice was inadequate in that regard—surely he bought you the most beautiful ring he could afford, so if everything else about the ring works for (metal, stone shape, style) then hold your tongue. Five years from now you can discuss upgrading to a diamond of better quality.