Personally Ive never liked Rarity/Spike. Rarity is a grown woman and Spike is just a baby! Keeping an open mind I suppose they might be able to get together someday, depending on how fast Dragons and Unicorns age, but eventually Spike is just going to get way too big for Rarity and will have to spend most of his time trying not to step on her.
Spike may be a baby dragon, issue his grow rate is slower than ponies. But, in pony standarts, he's almost same age as mane 6. Also they seem to be more young adults. And, maybe he can grow until he's enough big, and maybe his growing will stop enough time as to be few year together with Rarity without problems, later yes, he may be too big for her, but I think that's not an issue, actually. I mean as far as they love eachother. Like in this pic I love: [link]
Actually Dragons are Polymorphic in nature, as in their physical forms adapt to their environment. Spike's "baby" dragon form is more due to him living among ponies and the size he needs to be to survive.