Numbers: ordinals and cardinals

Question

I would be grateful if anyone could help. I'm confused about using ordinal and cardinal numbers in such examples:

If I want to get a larger size of shoes, what should I ask for?

Can I have forty-second size (42)?

or

Can I have forty-two size?

If both sound strange or artificial to you, how would you ask?

Next example:

If I'm seeking for room #10, my question should be

Where is the tenth room?

or

Where is the ten room?

or

Where is the room number ten?

(sounds most appropriate to me, still wondering if the first or second are Okay)

Thanks a lot in advance. If there is anything I can read, please feel free to share.

Answer 1

English puts the cardinal after the word 'size', like a size seven hat.

So for the first example, the correct expression is size forty-two:

Can I have size forty-two?

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For your other example, both of these expressions are idiomatic:

Where is room ten?

Where is room number ten?

It is possible to refer to a room with an ordinal, but it will indicate its location, not its number.

Jacob's office is the tenth room on the right.

Answer 2

The answer to your first question would be to say "Can I have a size forty-two (shoe)?". Forty-second is used as an indicator of a counting distance from some arbitrary other set of shoes, while forty-two is used to specify a specific shoe size.

Your second question has the same answer, for the same reasons. Asking for room #10 specifies a room that has that number on the door. Asking for the tenth room indicates you are counting 10 rooms from another room, in this case probably room #1. Both of them could have the same meaning, but if the rooms were numbered in a non-sequential way (rooms 1,3,6,8,10,4,7,2,9,5), then the 10th room from room #1 (in this example) would be room #5.