I have a house I am building currently in missouri, the house has a basement and the foundation is rectangular, I have a load bearing wall down the center of the basement made out of 2x4's. I decided to put a garage on the left side of the basement and was wondering how wide I can span a header? I would like to go at least 16ft if not 20' what I am asking is what will be the proper way to do this? 2x10 2x12 doubled tripled...etc? just want to make sure its correct I hope I gave enough information to determine the answer to my question if not please let me know, any help is greatly appreciated!

Not nearly enough information, and such things should be designed by a qualified design professional who has visited the site and actually examined all conditions which relate to the design solutions, or at least examined all drawings for the building for the same reason. It is much too dangerous to attempt to provide structural design services in a forum such as this, because of the risk that the information provided will be incomplete or incorrect.

I will say that generally speaking, conventional sawn lumber will be inadequate for such a long header, and the solution probably lies with such products as LVL's, or steel, depending on the loads that the header is required to carry.

I should have added I am not the one building the home I have hired a contractor that is doing it. I am just asking the opinion of the people on the board to double check that everything is properly done. I have hired a good well known contractor that i trust but I am the type that likes to double check and make sure my home is properly built.
What more information is needed?

Principally the load that the header would support is required, but there may be other issues that a competent design professional may need to consider and deal with. But most of all, the person designing the header should make himself or herself fully aware of every condition that would affect the design, and that can't be done in a forum. There are also ethical considerations beyond that...for example, anyone who does give such information should be licensed in the state where the building will be built, and there are heavy fines and other punishments for people who try to practice architecture or engineering without being licensed in the state. Not to mention that we, and others, make our living by providing such services, and I, for one at least, do not give away that by which I make my living, nor would i have much respect for another professional who might do so.

wow thats all I can say my friend is wow

Well, all I can say is that a header is a beam, and in order to design a beam, one must know the loads that the beam will have to carry. Since that information is absent, then one must either examine the existing structure, or examine the drawings for the structure, in order to determine the loads upon the beam. Ethically, one must examine those things in any case, because there is no guarantee that the information given in a forum is correct.

Further, people make their living doing such things properly, and a forum participant has no right to expect free services.

In your case, I pointed you in the right direction, away from conventional sawn lumber, and toward laminated veneer lumber or steel. That was all I could do. If you insist on free services, show a set of your plans to the lumber yard, and the manufacturer of the LVLs may have one of their engineers design the header for you. That is all I will say..

I don't typically agree with Richard, but it is true that design professionals should not work for free. The more design professionals give away advice/services for free the more someone else is not being nired to perform a professional service.
That being said, you have not provided nearly enough information. Additionally, how many stories is this house.

There was a great sequence on "Fawlty Towers" The british series with John Cleese operationg a hotel.

The local contractor wanted too much money to install a new door in a wall. Basil (John Cleese) Hired a laborer to do it cheaper. The end, of course, was that the wall was a bearing wall and there was no header installed. Very funny.

As a home builder, I always hire and engineer and make sure that they have the information needed to calculate the loads. That being said, I then look at it myself and compare it to past construction projects and apply my own judgement in an attempt to visualize the loads and where they bear.

First I look at the proposed assembly based upon my past experience. Then I look at similar assemblies in actual projects. I like to obtain building plans from other similar properties. By the time I get to the engineer, I have an idea of what he might come up with and I propose my own solutions to compare with his. Then we always add a little extra for safe measure.

From the inquiry, the gentleman was not sure. We don't like to proceed with anything until we have a very high level of confidence based upon facts and data.