If your connection to your ISP is PPPoE, the optimum MTU is 1492. DHCP is 1468. AOL DSL used to be 1400. VPN/PPTP is 1430. It's a mess :-) So, 1400 will always (er, almost) work. The oldest devices your packets may bump into could still be at 576. It's a mess out there :-)
If you're comfortable at a command prompt and you have time on your hands, you could work out your max MTU using the "ping" command.
ping -f -l 1472 www. p i c k a d o m a i n.com
The "-l" sets the buffer size.
The "-f" stops intermediate devices from fragmenting the packets.
You'll get an error message like "packet needs to be fragmented but...." (I forget the rest).
Reduce the 1472 in steps of 4 or 10 depending on your patience, repeat ad nauseam.
When you stop getting error messages, add 28 to the final number. That's your maximum MTU. If you want, you can try other domains dotted around the world.
Of, if 1400 works for you, make an executive decision not to try any of this
Edited to add wanton complexity to what started off as a simple post.