The size of the your portfolio banner will change depending on the device you look at it on. But a good size for your banner is 1200 pixels wide and either 310 pixels high for a normal banner and 480 px for a large banner.
Create your banner using good typography and good taste.
1. Create an Illustrator file 1200 px wide and 310 px high.
2. Open your Logo design project, select it, and copy it.
3. Past your logo into your banner.
4. Add shapes, text, a new layer with a background color. Be sure to leave the top of your banner empty for the website name and the text for pages that you add to be seen at the top.
5. File > Export > Export As your banner as a jpeg if it's done or if you want to bring it into Photoshop, use File > Export > Export As and choose Photoshop .psd for the format.
6. Put your banner on your website.