how to know proper length when changing a chain
 

Ok lets say you dant have the old chain to go by...I have seen YOUTUBE videos say,,,,just put the new chain over the largest rear and largest front ring and pull the chain tight then add 4 links...to me that doesnt seem like it would be long enough AFTER you thread it trhru the rear deraulier....

Short answer, it is for any modern slant pantograph derailleur.

Longer answer is that it might not be for the older designs where the pantograph is nearly vertical, such as older (pre 1975 or so) Campagnolo, Simplex, Huret, and Shimano eagle.

Modern derailleurs are designed so the lower pulley can rotate up above a straight line lower loop. On older designs, where that isn't possible measure big/big through the RD and add 1" after pulling all the slack out. (This method works for both new and old designs, and many old time mechanics do it this way all the time)

So run the chain big to big but DO go thru the derailuer..pull tigt and add an inch ?

Yes, One to two inches. On a modern derailleur you'll find yourself pulling the lower loop straight, so it's the same whether the chain is through the RD or not.

I must be missing something...my road bike has 105's and the derauiler is not straight when in low gear...its up and down straight,,,NOT parallel to the ground//

You're not missing anything. The big/big+ method confirms the shortest safe length for the chain. But the chain can be longer, and when using RD on gear ranges less than their full capacity, there's a decent amount of room for a longer chain.

Also, in low gear, your chain isn't on bib/big. Shift to that and you'll see the cage rotate to nearer horizontal. Then convince your self that it can go yet more horizontal by pulling the lower loop forward (keep the crank from turning) until it's straight.