Is there a Cisco box that performs (Ethernet) bridging over an IP network using for example BCP over L2TP?
It would be possible, I believe, to transport Ethernet frames over IP (maybe GRE?):
It may be possible, but officially unsupported.
If the MTU of the network is smaller than Ethernet's (1500) the frame will be broken down into more IP packets, reassembled at the other end so the complete Ethernet frame can be received.
Not so easy. IP Fragmentation is a layer 3 (routing) feature. Once you do bridging, layer 3 is out of the picture.
LTpv3 promoses to transport ethern over IP. In fact, is known to woork poorly.
So in short, do regular routing, or VPN, and everything will work fine.