How to increase the size of the cursor in Lubuntu 19.04?

Not from the desktop as themes do not support multiple sizes yet. There is a workaround it seems:

From the comment by sroracle to the bug report:

As a workaround, setting the XCURSOR_SIZE environment variable works. It should be noted that the Xcursor.size resource in ~/.Xresources or similar is ignored however. This issue should be looked at as out-of-box behavior on HiDPI systems is to have the cursor as small as possible.

Through the usual GUI, then no—probably not yet because the LXQT team has not added this feature. There is already someone reported this issue on their GitHub page https://github.com/lxqt/lxqt/issues/953.

@comodoro This is a valid feature request and is marked as so. No option for changing the cursor size exists in LXQt settings yet but there are other ways to do so; for example, see https://wiki.archlinux.org/index.php/Cursor_themes

For the meantime you can either download large mouse cursor to substitute it, or do it yourself—here is an example of someone who trying to accomplish same thing as you → https://forum.artixlinux.org/index.php/topic,847.0.html.