I am not sure if I get your question right! However, if I understand it correctly. I think it is a matter of dimensions (width and height). and using "ray" command you can change the space around the peptide.

https://pymolwiki.org/index.php/Ray

Marko Babić

You can use the zoom command in PyMol to scale your molecule such that it fills the window with a specified number of Ångstrom of buffer space around it.

zoom [ selection [,buffer [, state [, complete ]]]]

https://pymolwiki.org/index.php/Zoom

If you set the buffer to 0, you have no space around it, but you may want to specify

set orthoscopic, on

to turn off perspective, as this may interfere with the exact fit depending on the Z position of your molecule

https://pymolwiki.org/index.php/Orthoscopic

The github version of PyMol is open source, therefore free to use.

"PyMOL is a commercial product, but we make most of its source code freely available under a permissive license. The open source project is maintained by Schrödinger and ultimately funded by everyone who purchases a PyMOL license. Open source enables open science. This was the vision of the original PyMOL author Warren L. DeLano."

https://github.com/schrodinger/pymol-open-source

However, with this version you have all the hassle of having to compile it, including all the dependencies: https://github.com/schrodinger/pymol-open-source/blob/master/INSTALL

With the incentive version, you get a hassle-free pre-compiled installation package. In the free edu-version, ray-tracing is disabled (no shadows, reflections), while you can still use the png command to export images at any resolution desired:

png filename[, width[, height[, dpi[, ray[, quiet]]]]]

https://pymolwiki.org/index.php/Png

If you use PyMOL-generated images for publications in an academic/non-profit environment, you should buy the commercial version (99$ year)

https://www.pymol.org/buy.html#divacad