Any mom's here with a convertible car seat and a small/compact car?

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We're going to try to find a convertible or "forever" carseat soon but we're running into the maybe issue of size. I know they can be HUGE and with my husband's Mazda 3 hatchback I don't know if it's going to fit- I mean I barely do lol. Does anywhere know of any resources we could check on that; or experience? I know each seat has its dimensions online but you can't tell until it's really in there.