|I found these instructions online but I cannot test them as my work place block access to everything including Folder Options. Hope it works for you|
Maximize Notepad when double clicking a .txt file in Explorer, or by command line.
Apart from the above solution there are other ways to do it, and to do it for all txt files in one go.
To maximize Notepad when double clicking a text file, in explorer:-
1) select 'Tools' > 'Folder Options' > 'File Types'.
2) Click the Files Types folder and look down alphabetically for "TXT Text Document".
3) Select it, click Advanced > open > Edit and replace the "application used to perform action" box with:-
cmd.exe /c start /max C:\Windows\notepad.exe %1
For Start > Run you can use this command line:-
cmd.exe /c start /max C:\Windows\notepad.exe "C:\your dir\yourFile.txt"
Information about all this is here http://ss64.com/nt/cmd.html
I have XP Professional SP3, 32 bit.