Draw in PowerPoint, then
"Save As"
a Portable Network Graphics (.png) file, with resolution at 300 dots per inch (set via "Options" button in "Save As" dialog).Open the page (e.g. Slide1.png) with GIMP. Go to
Tools
Selection Tools
Rect Select
to select the rectangle you want. Then selectEdit
Copy
Save the image subset by clickingFile
New
in GIMP (makes a file the right size), then chooseEdit
Paste
then clickSave As...
in GIMP, and choose Encapsulated Postscript (.eps). This saves the rectangular region as a .eps file of high quality.