Concerns For Boa Enclosure

[coby]

Hello,
I'm new to the forum and I want to ask about an idea I have for my hypo red tail boa enclosure. She's about to be a year old but she is growing more quickly than any other boa I've had before. Right now she is in a 20 gallon long. I want to upgrade her to a 55 gallon tank (48"x13"x21") but I'm hesitant to do this because the 55 gallon isn't any wider than the 20 gallon, it's just two feet longer. Would my boa be able to grow in this tank even though it isn't any wider? I don't want to stunt her growth or keep her in something that is too tight for her. Thank you all.