I agree with the others - a 1lb or 2lb mold to start is perfect..... preferably one of each. Or, if you can find or build a mold that can do both, that's even better. My first mold, which was gifted to me 9 years ago and which I still use to this day, is versatile like that. It's built in such a way that I can do anywhere from as small as 1lb batches in it, or all the way up to 4 lb. batches in it. Also, I can use it in log mode or slab mode, it's safe in the oven up to 170F/77C, and it's completely collapsible. It was built by a fellow soaper who used to be a member here but hasn't been heard from in years (SoapmakerMan). The last time I checked, Dianna's Sugar Plum Sundries was selling a similar mold for $75.88. Here is mine (it's very well-loved as you can see):
Closed (If I want to do a fancy raised top on my soap, I can raise the cover up a little by resting a little bit of modeling clay on each corner of the mold to elevate it up, and still have the cover completely enclose the mold all the way around with no gaps anywhere:
Slab Mode, shown with a decorative silicone fondant mat lining the bottom, and heat-resistant mylar lining the sides (they are normally stuck in place better than shown when I'm soaping, but I just wanted to give you the basic idea for the pic). It will make 9-bars of whatever my preferred thickness if poured part-way up, and 18 1.25 thick bars if completely filled.:
Log Mode (sorry, didn't line the sides in this pic). I can do sample, 1 lb. batches of soap in either (or both) sides to make 4 slab-type bars (per side); or I can fill either (or both) sides up with 2 lbs. of batter (per side) and get anywhere from 7 to 10 bars of soap per side, depending on how thick I cut them, or I could do blocks of soap if I want.:
Collapsed (it's completely collapsible on all sides)
Although I have a handful of other kinds of different molds, this is still my most favorite one for its versatility.
IrishLass (I'd add my usual smilie, but I'm past my quota of images for this post, lol)