The handbook recommends using a 32M boot partition, but I find this to be horribly too small. This only allows for two kernels to be installed at one time. 64M or even 128M would be more ideal, and shouldn't be a problem with the availability of disk space these days. Reproducible: Always Steps to Reproduce: 1. Follow the handbook
I have two kernels installed, and lots of extra grub splash screens and various initramfs for graphical bootups. And even with all that, only 5.9MB is used for /boot. 32MB allows for storing lots of cruft--er, stuff. It's plenty. Anything more would simply be a waste of space. As we tell you in the document, it's only a suggestion. You're free to specify your own value. If you want to waste a whole 128MB on /boot, that's your choice.