I totally agree about making it bigger, or at least leave the option to eventually add on when you are ready.
We only planned to have about 5 goats, we have boer/percentages and made a barn that is 16'x12' out of pallets - I have a topic on it in the link that liz gave above, I love our barn, it's not fancy but we saved a lot of $$ making it and it is very sturdy and if we had done everything close together it wouldn't have taken long to do at all.
Anyway, my goal now is the fact we have more goats than we have room for. Well okay so we have 9 now, 2 will be sold, that leaves us with 7 - the goats we have now are fine, plenty of room.
BUT, 5 of them will hopefully be bred for Jan/Feb kiddings, 3 stalls, 1 stall is full of hay.
So I have to set up temporary stalls in the sleeping area!
Therefore, I now have to build a run in shelter before the girls start kidding.
So if you plan to have 5 goats, count on housing double, plus kids if you plan to breed
Make sure you have a good way to ventilate, it can get really stinky the more they have to be inside.
Plan everything out on paper and give yourself time to check it, if you know people who have animals/barns, get an opinion, or you can even post your layout on here
If you plan to have heat in the winter, find out the safest, and cost efficiant way to do it. Safety on electric <we don't have electric in our barn, afraid if we dig we'll hit the electric to our septic tank - so when I need electric I run an extension cord out to an outdoor surge protector that is mounted in the barn wall>.
Oh, and make sure no matter what kind of floor you plan to have that it's raised a little higher than outside to help keep the risk of water pooling up into the barn during heavy rainstorms.
Make sure your doorways are wide enough for a wheel barrow to fit through, and if you have a muck pile try to make it easy access, BUT I also recommend fencing it in, or something so your goats don't climb in it. we have that problem right now, and I plan to use pallets to box it in.