What car do you have?!
I didn't think ours was particularly tiny (Hyundai i20) - but after having DD we had to put her car seat on the passenger side of the back seat as tall DH needs driver's seat fully back and car seat wouldn't fit behind it!
DC2 now imminent and it's dawning on me that, short of chucking them in the boot, we'll probably need something larger -
I don't like big cars as a rule, so want something 'small big'
but would prefer to keep the DCs rear-facing for as long as poss which I'm assuming means even more space needed?
I know nothing about cars, and tbh the thought of spending hours researching, or even worse visiting showrooms, bores me to tears- so I thought I'd cheat by asking on here! 
Do you have something that works brilliantly for you??