Feel bad for turning down a present.

I'm going out today and put an outfit together and felt like it would look good with a leather jacket. I asked my mum if my sister is home to ask her if I could use her leather jacket. She wasn't so I tried to figure out what would look good instead. My 18th birthday is soon and my mum cam back to me with a box saying it was an early birthday gift. I was so excited because I've been wanting to buy a leather jacket but haven't got round to it.
I hugged her and thanked her loads and told her it was a thoughtful gift. When I went to open the box, my mum told me she got it in a size 16. I'm a size 10 (sometimes 12). I knew it wouldn't fit/look good but stayed excited as I opened it. I put it on and it was huge. It had no shape and just didn't suit my body. I tried to act like I was still thinking about it, zipping it up and down, twirling in the mirror and just going "...uhm... It's pretty but it's a bit big..."
We reached the conclusion that my mum will return it. I feel awful! I was so excited and so was my mum and we both ended up disappointed. Because I'm tall, my mum always gets sizes way too big to make up for arm length but size 10/12 sleeves fit just fine.

I feel really bad and was wondering if it was the right thing to do or if I should've just went with it.

why go along with it? whats the point of keeping something that looks wrong on you? thats kind of a waste. i doubt you hurt her feelings, surely she'd want you to have a perfectly fitting jacket. no idea why she got her size 10 daughter a size 16. a 12 i can understand but three sizes bigger??

It's not like you shoving it back into her face, you've still been appreciative about it so don't worry

Youve done nothing wrong dont worry about it

you only turned down the present in the particular size, if you made it clear that you wanted the exact same one in your size then it's fine