what is the minimum size of the /boot partition in ubuntu

[user1]

For years I have used a /boot partition of 100 mb, but now I cannot 
upgrade my software packages in ubuntu lucid 10.04, as I get the message 
that my /boot partition is to small.

I use to have as a minimum 2 following ubuntu versions (9.10 and 10.04 
just now) installed and running on my harddisk each of following 
partition sizes:

/boot = 100 mb 
and
/ = 30 gb

So Now I will have to resize both partitions, in order not to have to 
resize all the other partitions which I use.

So, what will the correct minimum size of the /boot partition for the 
future be?  :-)