i wouldn't see why not. There is room in a normal size taxi for 3 to 4 people anyways. when i went the 4th time to spain i went with 2 of my friends, and one of them was a bigger girl, and we didn't seem to encounter any problems, i didn't really think about it, so i don't believe there should be a problem.
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Spain Page!! Check it out!
http://www.geocities.com/missmadrid98