The Malibu bulbs i just installed from Home Despot measure .6in dia, x 1.4in long and fits EASILY in the lamp housings.
That equates to 15.24 millimeters dia x 35.56 millimeters long.
The bulb type is 194 (mainly refers to the type of connector on the base) So I go to an LED automotive light store and search to get:
http://www.autolumination.com/194.htm
Most all of those LED's will fit our standard housing. Its quite a selection, but check the dimensions listed for each bulb to be sure. I
f its is smaller than 15.4mm x 35.56mm then it should fit beautifully. I have a set on the way that are pushing those dimensions a bit, will report back with results. I know its not a recommendation, but its good info to know and gives you a range of types and styles to look at.
You can check here for the approximate lumens for each bulb style there in order to determine relative brightness.
http://autolumination.com/specifications_194.html
Hope that helps.