Make sure you publish or at least save everything you do at frequent intervals. You can always change it later.
Next go to your posts page:
[or to any post then near the top click on Design]
[or to any post then near the top click on Design]
Click on Template / Customise / Layout.
Choose one of these.
Then click on Adjust widths.
The top slider changes the main one under the Blog title [here A2 G324 and G325...]
The other one here changes the column on the right [here external blogs of interest]
When happy with changes click on Apply to blog [top right next to Help - again almost invisible]
