MTU 1500
Using Bitmask client 0.9.0rc4-dirty (325e513a84) on Ubuntu 14.04, I get an MTU of 1500. Some things work, many websites break (including https://leap.se/code) with connection reset or failing handshakes. Works fine when I set the MTU manually to 1371 (that's low I agree, I complained at my ISP).
Platform in use is 0.7 on demo.bitmask.net. This is related to #6745 (closed)
(from redmine: created on 2015-10-20, relates #6745 (closed), blocks #7545)