Open /Modules/ImageManager.ascx and change the 800 and 550 in the following line
<input type="button" onclick="showPopWin('<%=Page.ResolveUrl("~/ImagePicker.aspx")%>?&ct=<%=ClientID%>_imgHolder&cn=<%=imgPic.ClientID%>&rif=<%=ImageFolder%>&cif=<%=ImageFolder%>', 800, 550, null);" value="..."></input>