All Kernel image files are located in /boot. To find out which one you are currently using, run
uname -r

As en example, on my Xubuntu 12.04 machine, if I run

xuser@esr:/boot$ uname -r
3.2.0-53-generic

Now, to check in /boot:

xuser@esr:/boot$ ls -lha /boot | grep 3.2.0-53-generic
-rw-r--r--  1 root root 782K Aug 23 00:13 abi-3.2.0-53-generic
-rw-r--r--  1 root root 145K Aug 23 00:13 config-3.2.0-53-generic
-rw-r--r--  1 root root  14M Sep  9 16:46 initrd.img-3.2.0-53-generic
-rw-------  1 root root 2.2M Aug 23 00:13 System.map-3.2.0-53-generic
-rw-------  1 root root 4.7M Aug 23 00:13 vmlinuz-3.2.0-53-generic

As you can see, my kernel image file is called vmlinuz-3.2.0-53-generic is 4.7 MB big. The initial RAM disk it loads has 14 MB.

This will give you the size in MB of the full set if you run it on the terminal:

du -cm /lib/modules/$(uname -r) /boot/vmlinuz-$(uname -r) /boot/System.map-$(uname -r) /boot/config-$(uname -r)