Use the size YouTube recommends
YouTube recommends a 1280×720 image (16:9) with a minimum width of 640 pixels, under 2 MB, as JPG, PNG, or GIF. Custom thumbnails may require a verified account.
One idea per thumbnail
Most viewers see your thumbnail as a small tile on a phone. Show one subject, high contrast, and at most three or four words of text that add to the title instead of repeating it.
Check it at the size of a phone tile before you upload. If the text is unreadable there, cut it.
Match the title, then test
The title and thumbnail should make the same promise from two angles. If a video is not getting clicks, change one of them at a time and compare the click-through rate in YouTube Studio over the following days.
On Pro, the Creator Climb audit adds AI thumbnail notes for your top-priority videos.
Questions
What size should a YouTube thumbnail be?
YouTube recommends 1280×720 pixels at a 16:9 ratio, at least 640 pixels wide, under 2 MB, in JPG, PNG, or GIF format.
Should I put text on my thumbnail?
A few large words can help when they add information the title does not. Keep it readable on a phone.
How do I know if a thumbnail is working?
Watch impressions click-through rate for that video in YouTube Studio, and compare it with your channel's other recent uploads.