"Do you have a smaller shirt?"
Translation:У вас есть рубашка поменьше?
It's not wrong, but it works in few contexts.
«У вас есть поменьше рубашка?» could work the noun is the main piece of the information asked. I.e. unless you're asking "Do you have a smaller shirt?", with 'shirt' emphasised.
So, it could be used in a context like:
— У нас может быть что-то поменьше... 'We could have something smaller...'
— У вас есть поменьше рубашка? 'Do you have a smaller shirt?'.
In this example, «рубашка» constitutes the main piece of the information asked, and «поменьше» is a known information, it's already known you're looking for something smaller.
In most other contexts, «поменьше рубашка» doesn't work.
Also, the word order «У вас есть поменьше рубашка?» could be used in colloquial speech. In colloquial speech, the word order is generally less restricted, because the intonation helps to get the message through. However, written speech and formal language is stricter.
Duolingo generally doesn't allow non-neutral word orders in Russian. This is to make sure you learn the neutral word order first: it's better to learn the rules before learning how to break them. The general rule is that comparative degree of the adjectives comes after the noun.
«Меньше» is the usual word for 'smaller, less, fewer'. «Поменьше» implies 'smaller/less/fewer to some extent': 'a little smaller, a little less, a little fewer'. With shirts, «поме́ньше» sounds more natural because you usually need just a shirt that is just a little smaller.
At a store, wouldn't it be assumed that they do have at least some shirts, and thus есть wouldn't be necessary?
This doesnʼt exactly work this way. You can drop «есть» if existence is not the main idea of the sentence.
In «У вас есть рубашка поменьше?», you're asking about the existence of a certain shirt. You want to know if they have a shirt or not. You can assume they have some other shirts, but you can't assume they have the shirt you're talking about. You want to know if they have it.
In «У вас рубашка поменьше?», existence is not important, so you assume a certain shirt exists and you're asking if this certain shirt is smaller. I.e. «У вас рубашка поменьше?» will be translated 'Is the shirt you have smaller?' or 'Is your shirt smaller?'
I don’t think there’s a real reason.
All the translations are added manually, and some valid translations are sometimes overlooked.
You can use the Report button and select the ‘My answer should be accepted’ when you encounter this sentence next time, to make sure the course contributors receive a report. However, keep in mind that the course contributors get thousands of those reports, so it will take some time to be added.
This is too complex grammar too early. Without a larger knowledge of words & synonyms and their particularities - and with so bad translaters at hand; which as a rule happily confirm all wrong (!) exceptions - the value of time spent is zero: - at best!