why "速さ" instead of "遅さ"、etc


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.




As you say, it is a convention to use the more "positive" word in Japanese.
However, there are situations where we native speakers opt to use the "other"
word. I could think of two conditions that make using the other word sound
natural or even better.

1 Common knowledge:

This is when a thing's slowness, smallness, lightness, etc. is commonly known
to the general public and the speaker decides that it can be used as a premise
of a conversation. The slow speed of a snail （遅さ） is a prime example of this.

2 Contextual/Situational:

If you have to talk about the slow speed of a snail to someone who has never
seen or heard of a snail, you would need to first explain how slow it moves.
After explaining it, you can logically use 遅さ instead of 速さ.

Another example would be how "slowly" a Shinkansen train which is well-known
for its high speed could run on a snowy day. It can actually run more slowly
than a bicycle in certain places on a snowy day. In this case, you first
explain the weather condition and you can use 遅さ with no problems or
unnaturalness to the native ears.

Finally, allow me to correct your Japanese. You cannot say 新幹線の速さは何ですか as that
clearly is a direct translation from English. You would say
新幹線の速さはどれぐらいですか、新幹線のスピードは何キロくらいですか, etc. Since we are discussing 速さ/遅さ, I
probably should not be using the word スピード even though it is the more natural
word choice for us.




As you say, it is a convention to use the more "positive" word in Japanese.
However, there are situations where we native speakers opt to use the "other"
word. I could think of two conditions that make using the other word sound
natural or even better.

1 Common knowledge:

This is when a thing's slowness, smallness, lightness, etc. is commonly known
to the general public and the speaker decides that it can be used as a premise
of a conversation. The slow speed of a snail （遅さ） is a prime example of this.

2 Contextual/Situational:

If you have to talk about the slow speed of a snail to someone who has never
seen or heard of a snail, you would need to first explain how slow it moves.
After explaining it, you can logically use 遅さ instead of 速さ.

Another example would be how "slowly" a Shinkansen train which is well-known
for its high speed could run on a snowy day. It can actually run more slowly
than a bicycle in certain places on a snowy day. In this case, you first
explain the weather condition and you can use 遅さ with no problems or
unnaturalness to the native ears.

Finally, allow me to correct your Japanese. You cannot say 新幹線の速さは何ですか as that
clearly is a direct translation from English. You would say
新幹線の速さはどれぐらいですか、新幹線のスピードは何キロくらいですか, etc. Since we are discussing 速さ/遅さ, I
probably should not be using the word スピード even though it is the more natural
word choice for us.

