Macadamia nuts in Malaysian cooking
The only way the size really matters is if you're counting the nuts - unless a single nut is large enough to account for a fair proportion of the total. Weight for weight 1:1 should be good. Volume for volume 1:1 would be a good start but you may want to adjust to suit you in the future. 

If you were using larger nuts than the recipe called for, halving then (or chopping them) might be a good idea.