What Nintex Forms version You are using. Here (Nintex 2014 2.6) resizing the control by mouse results in showing more items.

Resizing the control by JavaScript describes Nintex Forms - Resizing the Choice List Multiple Selections control - Vadim Tabakman .

Thank you for your reply Manfred,

Unfortunately Vádim's solution doesn't seem to work for our Nintex Forms version, which would be Nintex Forms 2013 on-Prem version 2.10.0.2. First of all he uses a Choice control, where I use a List Lookup control. Although they seem to be the same, their dropdown list implementation seem to differ. Also, from personal experience it seems that in an update after version 2.9.x the dropdown list control has been changed in some way, such that a custom JavaScript snippet working in v2.9.x no longer worked after the 2.10.0.2 update (switch of <select> and <input> elements ????).

Finally figured it out myself. This can be done using custom JavaScript and utilize the NWF.FormFiller.Events.RegisterAfterReady event, because the control needs to be rendered first. Within this event you can specify your function to find the control's <select> element from the control's parent level (!!) and then change it's size property. Here's a code sample (yes, you could also put both instructions in to 1) where the default height of 4 items is changed to show 8 items (and a vertical scroll bar):

NWF.FormFiller.Events.RegisterAfterReady(function () {
    var myDropdownList = NWF$("#" +jsDropdownList).parent().find('select');
    myDropdownList.attr('size','8');
});