I am designing my first ever Conan (d20) adventure. The idea is that PCs are suppose to steal a boat or small ship for about 2-3 days sea voyage to get to a small cluster of islands. They are going to the isles from Argos.

The party will include a Pirate that will be probably only sailor in the party (of course I can enforce more profession (sailor) skills. The party will be 5 persons strong. So, what kind of vessel they need? Personally I don't know much about sailing or boats and I don't have any related books (and currently don't intend to buy them because this is possibly only a single shot adventure).

Then there will be antagonists, mainly a pirate crew with a small ship. They will be around 10 men strong, mostly pirates (chief is 2nd level and rest of them only 1st level) and maybe some other classes (like a soldier or something). They need a bigger vessel because they have more people and also because they are on their way from Zingara and thus have a longer voyage ahead (maybe a week).

So, I need suggestions for those two vessels with a bit of information (crew/passenger size and speed).

I would just wing it, and do combat as per the rules. You are afterall the games master. You know 5 Conan players will make mince meat out of 10 pirates, I would buff em up to around 30 pirates. Plus 10 pirates is a little low on the crew needed on most ocean going ships, since you don't have the books you wouldn't know that.
Yes, I will probably just wing it. Maybe I will just put the islands quite near the shore and so the boats doesn't have to be very big. I don't intend to have a vessel vs. vessel battle but the PCs will meet the pirates in one of the islands. I count on their ability to survive 1:2 odds although their characters will be first level too and none of us has any experience with the system :)

I plan to give a short write-up how the scenario went when we have run it...
If it's everybody's first time playing then 10 vs 5 should be pretty good odds. Keep a couple of sharks on hand for anybody who fails a Balance roll and falls in the water.

If things go badly for the players let the pirates gloat for a round. If things seem easy for the players make the ship heave and toss some of them to the sharks.

Make sure everyone reads the Code of Honor rules - it helps people get into the mood of the game. And if anyone has not read a Conan story (and you want to turn the night into a full campaign), swing by the local used book store and grab a couple of pocket novels to give out.
Well, I intend to have the physical encounter happen in the island, so no sharks there unless someone decides to take a swim :) Anyway I intend to give the PCs a chance to negotiate with a lot larger force and have a truce. I suppose it will hold on until the treasure (worth around 1000 sp in silver, gold and gems) is found. After this a combat is sure to follow. Of course I will try to keep the tension rising until the climax but if players decide to have the fight anyway without truce then so be it.

I am still designing my first character (2nd level pirate, who leads the NPCs). Boy, it takes some time to learn a new system :)