System Settings -> Workspace Theme -> Cursor Theme

Bottom right is a big dropdown button. Usually but not always with the options: "resolution dependent", 24, 48. I just checked with my machine and I also have a 36 size.

I setup a laptop with KDE/Plasma for a user with vision difficulties, one of their needs was a large cursor like they had on macOS. 48 was still too small for them on their 1080p display.

They usually need to use the Kwin zoom effect which gives a larger albeit pixelated cursor in addition to content around the cursor being zoomed including text.

I'm not sure if cursors are bitmaps or vectors, but if the default Breeze ones could at the very least support larger sizes that would be greatly appreciated.

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In addition to the cursor size, when applied, there seems to be a mix of the prior size and the new size depending on what the cursor is interacting with. On the system settings window, the resize cursors are the smaller original size and the default pointer is small when hovering over the windows titlebar area. This may cause some confusion or seem buggy. 

Unlike Fonts DPI this doesn't appear to be applied to new windows(existing Dolphin window had inconsistencies, new instance had no inconsistencies but used the small original size). It's corrected with a reboot, a warning dialog like Font DPI presents could be useful to avoid user confusion.

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TL;DR: Please consider supporting larger sizes like 60 and 72, if cursors are vector based a custom size would be great. If cursors are bitmaps but use vectors as source assets, a tool that could resize and rasterize the theme could address that too for supported themes?