Looking for peoples thoughts about top tube length on a DH bike.
Do you go for the same size top tube on your DHers as you do on your trail bikes?
Examples and heights would also be helpfull.

toptube length is completely useless on a DH bike I’m afraid (as you’ll never be riding it seated). What you really want to do is look at the reach figures instead.
For me, the reach of my DH and trail bikes are pretty close – the theory* being that there’s less to get used to when jumping from one to the other

*the reality is somewhat different

OK.
I see what you’re saying.
I suppose the TT length will be affected by the much slacker seat angle of a DH frame too.

Looking at my trail bike it’s a large with a 414mm reach.
A DH frame I’m looking at comes in a small with a 400mm reach or a medium with a 430mm reach.
What else should I be looking at size wise? As my trail bike falls bang in the middle of the two.

depends…

Do you like going flat out through rough chatt?
or
Do you like playing.. ie. manuals, jumps, pingin out of tight corners?

Every DH bike has been in line size wise with my trail bike, I ride a large mojo and a large Wilson. Long TT and reach, low stand over and slack head angle job done for me.

How tall are you?

414 is pretty short reach for a large frame? That’s pretty short for a medium too.

It’d be more for playing about and jumping.
I’m 6′ with a 32″inside leg.
Another of my bikes has a reach of 420mm

ETT is particularly useless on DH bikes not only because of the slack seat angles but also the tall stack height, so the projected horizontal line from the headtube intersects the projected seat tube way back over the rear tyre. What length stem is on your trail bike?

Also, how you set your sag affects the reach – more rear sag than front (as you’d expect on a DH bike) shortens the actual reach in use.

5’11” 33″ inside leg here and prefer small DH bikes now for just that reason.
But my trail bike is a dirtjump hardtail and I still like to ride BMX quite a bit.

Downtube is the same length on my DH and DJ frames (ie. reach right about the same) so if you ride your trail bike in the same manner you ride DH (I do) go the same size.

50mm on one bike and 40mm on the other with the longer reach.

At 6′ i’d be looking at a large.

Im 6ft and had a medium Spesh big hit as my first DHesque bike. I always have large bikes but a friend advised a medium would be better for learning to ride DH tracks and he was right. I now have a Large Banshee Legend, its alot more physical getting it round slow corners but when its steeper and or faster the longer size really helps but it takes more manhandling to get the most out of it. FWIW the toptube on my heckler is an inch shorter but its not really comparable.

Id say the small frame 400mm reach is too small, a few friends have had too small Dh frames and there knees have been banging stems as youre over the front & standing up alot more than youd think, being 6ft and riding a small frame doesnt make sense.