If I need to choose between,

Black 3/4 leggings - High waist tights
and
Gym Tights Black with Red Banding SUPPLEX Fabric For Comfortable Fit

I'll name my URL,
buymystuff.com/black-3-4-leggings-high-waist-tights

and in the title, I'd write "Black 3/4 Leggings - High Waist Tights" and the description of the product (and also on the meta description) I'd have, "Gym Tights Black with Red Banding SUPPLEX Fabric For Comfortable Fit"

And do go to Google Keyword Planner to check what are the most searched keywords for the product above in your country. You create a Google Adwords without a credit card, watch this tutorial, https://www.youtube.com/watch?v=6n8PWPDf4kk&t=26s

Hope this helps.

The length of the URL has very little relevance to rankings. I prefer to use shorter URLs for better readability and user experience. Long URLs are impossible to type in manually, where, if necessary, short ones can be.

The length makes no difference. However, the keywords themselves need to be things that are being searched for regularly AND that you actually stand a chance of being found for (ie you're on or pretty near to page 1).

Google Keyword Planner is a cool (and free) way to find out how many people are searching for your keywords every month. To check out the competition, just search for various combinations of keywords in google and see what sort of sites you're up against. There's more to it than that but broadly iif you see page 1 locked out with really massive sites then that's going to be an expensive keyword to target.

In theory if you stuff in more keywords then you stand more chance of hitting on something google likes. But if it looks unnatural to users then you'll have a disadvantage from that point of view.

Best approach in my experience is to research keywords thoroughly to find any big opportunities (ie high search volume/low competition). Target those opportunities in particular but otherwise just build content that looks naturally good to your users. A very large proportion of google searches are unique so you can rely on the fact that good content and good backlinks will get you hits!