Bring up Help and Support (from the XP menus). Search on "create custom startup files for an MS-DOS-based program".
You should be starting your dos-based programs from a shortcut. Once having the shortcut, you can RIGHT-Click the shortcut, to get Properties, then select
(click) the MEMORY tab. Depending on your program's needs you can set the expanded or extended memory size.
I usually leave mine set at "Auto". But you can also set a specific amount.
The choices are listed in terms (multiples) of 1000.