You can change the web rendition's dimension by modifying the Web Enabled Rendition step in the DAM Update Asset workflow model. You can change the argument in the Process tab by specifying a custom dimension (for example, 1920 x 1920).
However, after you save the DAM Update Asset workflow model, an uploaded image still creates a web rendition at 1280 x 1280.
This limitation happens while the image is buffered in the cache. There is a hard-coded value in - 'Day CQ DAM Buffered Image Cache' configuration (felix's console) under the 'Max Dimension' property. You can further change the default 'Max Dimension' property from 1280 x 1280 to a higher one (for example, 3000 x 3000), so that it supports the custom dimension defined in the workflow process for the web rendition.
The new image that is uploaded to DAM creates the web rendition with the custom dimension.
Note: You can't define a web rendition's dimension that is larger than the original (for example, a sample image of dimension 1280:1280). The workflow doesn't generate a larger rendition, such as 2600 x 800, as it causes a distorted image. If such a use case happens, it renders a web rendition using the original dimension of 1280 x 1280.