*Don't Do This* warned ya...
I'm not sure if there is an "Official" way of doing this, but... this worked for me.
If you go to your SolarWinds folder on your server (mine is c:\inetpub\SolarWinds\Orion\NetPerfMon\) and look in that folder for a file called NPMChart.ascx.cs.
** Make a backup of that file ** Seriously, make a backup of that file.
Edit that file using your flavor of text editor (my choice is notepad++).
Around line 441 you will find a line which reads return 640;
Change that to whatever you want your default size to be, I chose 1024 because powers of 2 are fun (I'm going to guess you have to keep it below 4 digits as thats one of the rules on the textbox size).
It worked for my installation BUT it might not work for yours and if you break it, well, you know the rest of that statement. Also, I'm pretty sure if you get a major update or something like that, this change might disappear.