Large size gives the following for any large creature:
-1 AC, -1 BAB, -4 size, +4 grapple checks, +4 bull rush checks
They can wield large weapons 1 handed, and huge weapons 2 handed.
Their armour costs and weighs more.
For 5x5 large creatures, they have a 10' reach.
For 5x10 large creatures, they threaten 2 more squares and usually are 4 legged animals (+ 4 bonus vs trips) and a greater carrying capacity.
There is NO standard for Str, Con, or Dex for large creatures. But as an average, large creatures usually have a bit higher str, sometimes a higher con, and usually a bit lower Dex. These questions should really be addressed by your race design. Check the MM for more rules on advancing monsters sizes.