This is a customized size ads, so you cannot generate tag normally from the AdSense interface. You must implement the following code and traffic to your site via DFPSB ad server only. You can register DFPSB for free here.
Steps to follow
1. Create custom channel and name your custom channel according to your preference. (should be named with something called "custom sized-300x800" to make it easy to remember)
You do not have to add any ad unit in this custom channel. Then copy the channel id number and replace it with "CustomAdChannel" text in the code below.
2. Adjust the width and the height according to your preference and change the width and height parameter in the code below. Please keep the height at least by 280 px in order for the image ads to show on the page.
google_ad_client = 'ca-pub-XXXXXXXXXXXXXXXX';
google_ad_channel = 'CustomAdChannel';
google_language = 'en';
google_ad_width = 300;
google_ad_height = 800;
google_ui_version = 1;
google_color_border = "#CCCCCC";
google_color_text = "#333333";
google_color_link = "#940F04";
If you want to make more decoration, please read this Custom Ad Format Documentation.
3. After that you can create a new ad unit in inventory tab of DFPSB according to the size you preferred and then copy the ad unit tag to paste into the source code.
If you don't known how to get started with Google DFP, watch the following video:
4. Trafficing customized size AdSense ad line item by creating a new line item and then choose "AdSense" as line item type. Target your line item to the new ad unit you just created.
5. Copy the edited customized size ad tag above (after adding channel id and change the size) into creative code field during uploading creative stage.
6. AdSense should be live on your page!
Creativity in Display Ads [Video]