Multiple routers connected directly -- MTU concern?

I have a DLink VOIP router, connected to a Netgear Wireless Router,
connected to a DLink Wired router. My PPC uses the W-Router and PC
uses the Wired router. I use both direct Internet and VPN tunneling
at different times.

Since I have three routers connected to eachother - are there any
concerns regarding MTU size?

Right now I'm using three layers of NAT... all needed for different
purposes of security...

Any other concerns?

Thanks