AFAIK there is no concept of TTL (time to live) in openvpn; I don't know of any way to alter "the TTL" in the server config.
If dropped connections are your main concern then I'd start out with
fragment 1400
(for UDP based connections). For TCP based connections I'd use
tun-mtu 1400
I'd avoid messing with the tun-mtu unless you absolutely have to.