Microworld Games have a selection of 6mm walkers in their Kreen range.
http://www.microworldgames.com/collections/6mm-scifi/KreenMechafront are probably my favourite 6mm walkers, they are of a similar size to most of the old Battletech range. They don't seem to be currently listed on his site, but they may be waiting a general release once the KS is complete.
http://www.mechafront.com/page/page/8798042.htmAs for 10mm ranges, I'm not sure if you're aware of them but these two ranges are well worth a look.
Hawk Wargames, Dropzone Commander has a selection of walkers that could suit 6mm. The PHR, Shaltari, and Scourge all have potential.
http://www.hawkwargames.com/pages/dropzone-commanderSpartan Games' Firestorm Planetfall has a variety of large Mechs that would look at home in 6mm. My favourite would have to be the Terran Alliance
Odin Class Leviathan. Although the Relthoza
Varisei-Kei is definitely memorable.
http://shop.spartangames.co.uk/SearchResults.asp?Cat=2015Hope that gives you a start! There are some awesome ranges out there, and I'm always stumbling on new 15mm & 28mm figures that suit 6mm with little to no work.