you need to edit this file
C:\Program Files\PokeMMO\data\themes\default\ui\inventory.xml
line 27 and 28 only edit the numbers don't change anything else
<param name="minWidth"><int>430</int></param> for width 430 is default
<param name="minHeight"><int>700</int></param> for height, i forgot what default is i made mine bigger xd
try whatever you like and dont blame me if you break something