-Silver- very nice
construct is rather quirky in that sense, that making every animationframe in power of two will actually NOT fit well in the final spritesheet because it automatically adds an extra pixel,.. personally, i find it a big mess to figure out the best fit, theres no feedback on how your assets will be distributed, except checking the final export...
Bigger the better is good but you let it to the customer to handle the best resizing, wich is not always good for everyone, will it actually look good at every size? idealy i want to give the original, (the biggest you want to go) and then a couple of steps, so the enduser can choose the best size/quality for the project, again ideally, because this all takes time, you can also take the approach as what do you find the best options for your assets...
The same for how many frames..., its easy to give someone a complex 50 frame animation, but for mobile and online you want that to be optimized, the size of your art x amount of frames can add quickly to couple of mb...
600*600 is pretty big and thats really HD for a character, a couple of options down from that and your good i think,.. good luck with your assets 
sman118