Elar,
Kevins advice is about how to setup a system printer for a specific default sheet size in the system printer configuration file (printer.plt), not how to switch between different printers.
I don´t know about a method to automatically switch between printers, if this is really what you want to do. I can imagine that this might be useful switching between a large format printer and A3 or A4 laser printer to produce a reduction of an A0 to an A3 sheet.
If you do not really need to switch between printers but only sheet sizes, be aware that MicroStation automatically tries to match the sheetsize name of the sheet and the sheetsize name of the active printer. This will be sucessful if they are the same.
For my company I have set up a set of standard sheetsizes (DIN) and some oversize formats and implemented these in the sheetsizes.def and the printer configuration files. So MicroStations matching produces predictable results with ease.
In general I would advice you not to print directly to a printer but to a plot file. Either HPGL or PDF are good solutions (depending on your overall workflows and hardware). It will automatically produce a digital archive of your plots and gives you the opportunity to check for errors before you waste paper. Also good viewers, such as ViewCompanion Pro for HPGL or Adobe Acrobat Reader for PD,F give you the opportunity to print reductions of your original large format plot files to an office printer. Another benefit is that MicroStation printer configuration files for HPGL an PDF are much more customizable than hardware specific drivers.
Before you start setting this up, be aware that there have been major changes in printer configuration files between V8 and V8i. Although legacy configuration files are supported, the new ones are recommended. So you want might to start with the V8i method right from the beginning.
HTH
Here is a line I add to my .plt file:
sysprinter /form="Ledger"
--Kevin