I am pretty sure that when native speakers talk about *speed*, they say "速さ", "広さ" for *width*, "大きさ" for *size*, etc. Am I correct about this? If I am, then: 

Why not say "遅さ" instead of "速さ", "狭さ" instead of "広さ"? I suspect that when you convert comparable adjectives to nouns, you should use the greater of the two. However, surely this must just be a convention, right? So,  

"新幹線の速さは何ですか？" // wrong?  

"新幹線の遅さは何ですか？" // wrong? 

what about:  

"snailの速さは何ですか？" // wrong?  

"snailの遅さは何ですか？" // wrong? 

And finally, are there cases, if there are any, where it's natural to use "遅さ", "狭さ", "小ささ", etc.? 

thanks.