Yup, the shape will be funny for a while but it will grow into a normal colony pretty quickly. I think they start looking normal faster if the cut frag has a bit of the top so it has some polyps to start with, though I'm pretty sure I've cut frags off the side or base and they were still able to grow out without issue. (I don't have the "tyree" toadstool, My exp is referring to a couple of similar sarcophytons.)
For a first time, I'd suggest something along the lines of a 1" / 1/2" slice. That way your not taking a whole lot of tissue off the parent, but the frag is still large enough that it will get attached and growing pretty quickly.
When you go to attach, make sure you don't rubber band the frag too tight or it will slice the frag in half and or tear it off of the toothpick. If you just rubber band it to a rock without a toothpick and it's a little too tight, you usually end up with a double header frag (kind of cool IMO).
If you feel ambitious, you can slice a big chunk off the crown and cut it up into several frags. You can even cut the entire head off and slice it up like a pizza- the original colony will grow a new head. (IIRC check out garf.org for a video or something showing this)