With a wheel off your trailer , you measure from the center of one lugnut hole to the far edge of the 2nd lugnut hole ( you skip one hole) this will give you the spacing measurement.( It would be written like this for example
6X5.5x15 the first 6 are the amount of lug holes ,the 5.5 is the measurement you just found ,and the 15 is the dia of the the tire seating area of the rim. For back space you run a streight edge across the rim(rim only ,not the tire and rim.) and measure the distance fron the rim edge of the streight edge to the plate that has the lug holes. You now have (as example) a 6x 5.5 x 16 with a 2.5" offset. Hope this helps.