Hey! I can relate, I have tried the Nano Speedy in store before in bright colours (particularly Stardust) and I agree it is tiny though it fits a lot. I'm average height and it looked like a child-size bag on me, too. It really is a personal taste thing, because I love how it looks on others, but i felt uncomfortable wearing it myself, yet I own several small bags and have no problem wearing those. I also have a Keepall XS and am more used to its longer shape so that may have played a part. *I don't think it looks like a child's bag on others, btw!!
it's a personal thing for me when I wore it myself!
It may have looked less small if I'd worn it with tshirt and shorts or similar casual summery outfit, but even then. I probably would have found the proportions weird on me. This is all personal taste at the end of the day, it is a really cute bag and useful and all. But, now that I've fallen in love with another bag, which was instantly lol, I am reminded that when you buy something luxe, you should really love it- if you're having these insecurities about your new purchase, maybe you should return it and save the funds for another style that gives you confidence instead.
Colourwise the Lagoon I'd love to try clash it with metallics actually - I tend to use colours as neutrals. It reminds me of the Stardust lilas colour. Try it with like, silver boots or jewellery, clash it with chunky chain jewellery, try faded grey denim or other denim shades- though maybe not black in case of colour transfer- wear it with your favourite sneakers or sandals, or a long coat if it's winter, etc. I think the Lagoon works well as a colour pop bag and goes with a lot of potential different outfits, but it depends on your style. And you should feel confident wearing the bag, no matter what outfit. A bag shouldn't make you question yourself.