Time and time again people ask me this same question - how can you guesstimate the size of a project.
Here is the first thing I always do - use the "Tao" method (also known as the "snowflake" or "Fractal" method) - please note I'm keeping the below simple and a bit naive! - you need way more to build such a complex product:
Say I want to build a massive multiplayer online game. This is general statement of the end product we'll call level one.
What do I need to realize this? High level items only - the list shouldn't be longer than 5 or so items, otherwise you know you've started on details. Lets say I split level one into the following: a) I need a dev team, b) a bunch of servers, and c) someone to take care of the servers. Keep level 2 very basic (although maybe not this basic).
Now level 3 is to split each item in level 2 further into detail.
For the dev team I need i) leadership ii) programmers iii) graphics guys iv) sound guys and finally v) story and level designers.
For the servers I need i) web servers ii) database servers iii) backup servers iv) test server.
(...and so on for level 2).
Now I continue onto level 3, going into even more detail, but still keeping it pretty rough. Example using the programmers item from level 2: I need network programmers, database programmers, 3d graphic programmers, general game engine programmers, sound programmers, and possibly tool programmers (the guys who build the level editors, etc).
And I do that detailing for each item in level 2.
Then I start off on level 4, detailing each item in level 3 a bit further...
And then level 5... etc.
Once you have a level of detail that you feel is trivial enough (for our complex example you probably hit at the very least detail level 10), you can start putting time and money estimates to each item.
The total of which will give you a pretty good idea of what you are in for...
Note: Even a simple web app will have 5 to 7 levels of detail iteration.
Sadly there isn't really an off the shelf product that you can use for this that is easy to use - I've used everything from Excel (easy enough to use) to MS Project (bonus: you already have the start of your WBS).
And please don't think you need to do all the detailing yourself (unless you are a one man team!) - get subject matter experts to help you out with the sections you know they have experience in!
Hope the above makes sense!