Comment 7 for bug 1831736

Sizing:

$ sudo du -sh /boot/* | grep -e grub -e 5.3.0-18
231K /boot/config-5.3.0-18-generic
8.0M /boot/grub
81M /boot/initrd.img-5.3.0-18-generic
4.5M /boot/System.map-5.3.0-18-generic
11M /boot/vmlinuz-5.3.0-18-generic

This is desktop system, amd64, with all microcodes, and linux-firmware, and signed grub.

For three kernels that brings my system to (11+81+4.5+0.2)*3+8 = 298.1 MB

Which is still within reasonable /boot sizing and need not to be bumped.

However, there are potential disk space saving that could be made:
 * on securebooted systems /boot/grub contains 8MB of modules that cannot be loaded at runtime
 * config is informational only, and is not strictly needed at boot
 * System.map is not strictly needed for boot
Which could save (4.5+0.2)*3+8=22.1 MB

Subiquity creates /boot at 1GB.
Ubiquity/partman-auto aims for (512 1024 768)
All are still reasonable for up to 10 kernel versions.