EEG spatial source resolution is within a few centimeters [depending on some parameters]; therefore, having a source space resolution within um or mm will not improve a lot the accuracy of your model.
In other words, having 10 sources or 100 sources within 1cm2 of brain will not really improve your localization, since in both cases your source will be within this 1cm2 area.
Using these 300k sources requires a lot of RAM memory both for the forward and for the inverse problem computation, and you also need more space to store the results.
If you still want a higher resolution than the recommended 15k source, you can use 50k or 100k, this could work on your computer.