It is quite simple to force windows to open whereever and at which size you wish them to, provided you're using KWin (which in all probability you do):
Open the application and then click on the application menu (with Firefox, for example, the 'Firefox' logo) at the top left.
Choose More Actions > Special Windows Settings.
Go to the 'Size and Position' tab and activate the option 'Position'.
Set the first field to 'Apply Initially' and enter '0,0' in the second field. Those coordinates will ensure that the application always initiates at the top left corner of your left-hand monitor.
(If your main screen is on the right-hand side, you'll have to choose the appropriate coordinates, of course, which depend on your screen sizes; just start out by using '0,1000' ... you'll figure it out from there.)
Have a closer look at the other settings on that and the other three tabs, too ... the nice thing about KDE is that you can fine-tune absolutely everything to a fare-thee-well, if you have a mind to.
If you can't explain it simply, you don't understand it well enough. (Albert Einstein)