I am a tech.
The Britax Marathon sits up higher on a base (typical of most convertible seats) and does have a learning curve for not hitting children's heads on the vehicle roof when you are not used to it... especially as kids get older/taller.
With your daughter's age/size, I would actually recommend returning it and purchasing something like the Graco Nautilus or Britax Frontier. Both harness to 65lbs and 80lbs (respectively) and then turn into *great* boosters later on. They both have nice tall top slots, so the harness will not be outgrown in height before they reach a good booster age (5 - 6 years old). The Frontier has a 9 year expiry date. The Nautilus has a 6 year expiry date on the harness and high back booster and 9 year expiry on the low back booster. Both should be the last seat your daughter needs.
The Frontier is the same width as the Marathon (even though it looks bigger

). It has more interior shoulder width room than the Nautilus. We personally have the Frontier for my just turned 3 year old and he loves it.
On a side note... I see a few people stating their children are small and will be in their seats forever. Both my children are lightweights, but they are on the tall side. They outgrow seats by height long before they are even close to the weight limit.
With that... basic rules for when a seat is outgrown...
A seat is outgrown forward facing when *one* of the following happen...
1. Weight limit exceeded.
2. Shoulders go above the top slots (harness needs to be at or above the shoulders).
3. Tips of the ears go above the top of the shell.
A seat is outgrown rear facing when *one* of the following happen...
1. Weight limit exceeded.
2. Head is within one inch of the top of the shell.
Hope that helps.
