First You will be fine with a 5 gallon carboy. The beer will only be in there a week or tow and will probably create enough CO2 to blanket the beer. It is quite common to secondary in a 5 gallon carboy.
A couple things about beer color. First beers look much darker in a 5 gallon carboy. Even Budweiser would look surprisingly dark. Look at your beer in a glass for a more accurate evaluation of the beer's color. Second Extract brews with partial boils will be darker than what you are shooting for. The concentrated wort darkens during the boil. The only way around this is full boils which require a very large pot or you can also do late additions of your malt extract. Less time boiling = less color addition.
EDIT: Deathweed and Deathbrew did a fine job answering your questions before I did.