Apple Watch sizes help please

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I want to get an Apple watch for DS for Christmas but want it to be a surprise. He is 15 and very slim (small wrists). I'm assuming the small size 38mm would be best, rather than 42mm but confused as to whether this relates to the band, screen or both? On reading one review online, it referred to the small 38mm being a "women's"watch. Please don't need to debate the political correctness of this! Just don't want to get the wrong one as 15 year old boys very sensitive to such stuff but good deals online just now so any advice appreciated.

It's not a women's watch. The smaller size will be fine. If you're getting a series 5, the sizes have increased anyway to 40 and 44, so even bigger now. My female friend has the smaller one. I have the larger one. You actually cannot tell the difference once they are on.

Oh and the size relates to the screen. The band of the smaller one fits wrists of 130-200 mm. Can you measure his current watch strap when he's asleep or in the shower?

Thank you @Incacat2. That's helpful. I am unable to get out and physically view them right now so reliant on internet.

Myself and two boys (12&15) all have the 38mm. They came with two different size straps so ds2 wears the smaller one and ds1 and I wear the larger. The 38mm will be fine.

@Tatum1234 thank you. Watch ordered!