The ideal twitter cover photo size is 1500 pixels in width and 500 pixels in height. When designing or using a twitter cover photo, you need to plan the design keeping the profile picture in mind.
The problem with Twitter guidelines is that even if you follow them, the images get cropped. You could use the guidelines below to match the Twitter profile without difficulty and display in the right manner.
The dimensions are specified before is 1500px by 500px. You can download PSD templates from this post. Twitter accepts only JPG and PNG files.
Guidelines for Twitter Cover Photo Size
Keep the sizes to 1500x500px at 3:1 ratio
Keep space for profile display picture at the top and bottom
Keep image size below 2MB.
If you find it difficult to design in photoshop or illustrator, you could use the canva tool to design your twitter cover image in the right size. You can use the twitter cover maker here
There are a lot of templates for twitter headers available on the platform.