At ground/lower levels going bigger results in going more docile, it results in slower responses ..
At high levels this may not be noticed to much..
Simply making the model bigger may work, but if the all up weight goes up in proportion to the bigger wing area you wont actually achieve a better performance
scaling up.. so dont scale it all up , first try increasing the wing chord, then the span.. remember that this is adding more weight, so some more power will also be needed.. extra area is needed in thin air to get the same lift..
power itself, the thrust of your setup will be lower, with the air being thinner ....So you will need to use bigger props than you would at low altitudes to get the thrust back up to what the model needs...
get a watt meter.. try bigger props but you must keep the draw in the spec range for your motor.. if that doest get the thrust up enough go up on the motor size..
Ideal performance ... aim at over a 1:1 thrust to weight for all models with higher ratios preferred... 2:1 is my target for any model,,, the throttle is always there to keep the model in check..
good luck...