I have 50 amp circuit breaker for double wall oven. Wire from floor is al shielded so I bought 6-3 al shielded but when connecting at junction box, the floor wire is thinner than 6 gauge (8?). The oven also has thinner wire so can I splice in 6 gauge or should I use same gauge as existing from oven and floor?

What are you splicing and where?
Was the wire from the "floor" not long enough?

You are describing this scene like we are there looking at it.

Also, what is "al shielded"? Do you just mean cable?

Sorry, there is aluminum shielded cable coming up from the floor and it is not long enough. The junction box for the wall oven hookup has to go in the upper right rear of the cabinet (tech support said had to go there and also oven depth for lower 2/3 is almost up against cabinet). The cable from the oven is not long enough to reach down underneath oven. So I'm tying in 6-3 aluminum shielded cable (to match the oven cable and the wiring from below) with a junction box below the oven platform where the wiring comes up from thr floor and another junction box in the upper right of the cabinet.

The cabling from the oven is bare ground, red, black, white and the cabling from the floor is red, black, white and green instead of bare, all are copper.