Short answer
About 60 characters for the title and about 160 for the description is a good target. Google actually cuts by width in pixels, so a title with wide letters is cut sooner. The free SERP preview tool on this site measures the width in your browser and shows how it may look.
Free tools for this
Characters are a guide, width is the rule
Google measures a title in pixels, roughly 600 on a desktop result. A title full of narrow letters fits more characters than one full of wide ones. So 60 characters is a sensible target, not a limit. If the end of the title matters, keep it short.
Titles that work
Put what the page is about first, and your brand last. Say it the way someone would search for it. Avoid stuffing the same word in again and again. Each page needs its own title, because identical titles across a site compete with each other.
Try it now: SERP and Social Preview → · Character Counter →
Descriptions that work
A description does not change your ranking by itself, but it decides whether the person clicks. Write one or two clear sentences saying what the page offers and why it is worth opening. Include the main words the person searched for, since Google bolds them. About 160 characters is a good target.
When Google rewrites yours
Google may show a different title or description if yours is missing, too long, the same as another page's, or does not match the search. You cannot force it. The best defence is a clear, accurate one that describes the page.
The share card
When a link is shared, apps read Open Graph tags for the title, description and picture. Use a picture of about 1200 by 630 pixels. Without these tags, the apps guess and often pick a wrong picture or a stray line of text.
On Shopify
Edit the title and description for each page, product or collection in its search engine listing. Your theme writes the tags from that.
Check the pages that earn you clicks first, then work down.