When selecting a dwarf nectarine or any dwarf fruit tree. You will want to distinguish between genetic dwarf trees and trees that are dwarfed due to a dwarfing rootstock. Both options have merit, but genetic dwarf trees will often remain smaller than rootstock dwarf trees. Also rootstock dwarfs will need to be pruned yearly to maintain their smaller size. Genetic fruit trees often only need occasional corrective pruning.
Three genetic dwarf cultivars of Nectarine (
Prunus persica var.
nucipersica) that I am aware of are 'Arctic Babe', Garden Delight', and 'Golden Prolific', but there are doubtlessly others to consider.
Prunus domestica 'St Julien' is actually a plum rootstock, but it is also a rootstock that is commonly used to make standard peach and nectarine cultivars behave as semi-dwarf trees.
For more in-depth information, consult with the University of Georgia Agricultural Extension (
https://extension.uga.edu/) to find out more about preferred nectarine cultivars and cultural practices for your region.