another game resolution question

guys, i'm trying to:

1-create a game in the 16:9 ration advised by Scirra (my game resolution is 800x450)...deliberately selected max 800 width to ensure the game can be submitted at the Scirra arcade

2-i have imported png art characters that look sharp/clear/crisp at the 800x450 res

3-but when the game is scaled up to play in the entire browser (tried both "Letterbox scale", and "Letterbox integer scale") all the characters look like crap, with jagged edges.

so, my questions are:

1-what is best game resolution, is 800x450 or 16:9 really ok (i want to be able to submit to Scirra arcade)

2-if i want the game to expand and play in browsers with higher resolution, how can i do that without getting the horrible jagged edges? the original art quality is awesome.

3-can i arrive at a solution by going at the opposite way? create the game at maximum resolution 1080p HD and have the game scaled down and played at 800-width (so i can submit to Scirra arcade)

guys, i reallyyyyyyyy need help on this. Construct 2 has many great advantages, one of which is playing on multiple devices...which is near impossible with this resolution nightmare. please help.

Yes, point 3. I would go with standard HD and scale down. You will lose detail on, but I guess it's better to show less of good quality than more of bad right?

thanks a lot briggybros...i have a follow up question:

i know how to create the entire game in 1080p HD; but how can i scale it down for scirra arcade (max width 800)? what settings do i have to use?