Judging by this table, the first few SKU levels are pretty limited, particularly, in the max memory. I will probably get an error if a semantic model exceeds these limits. But without waiting for an error, how do I check the size of my semantic models? Could not find this anywhere in the settings or documentation.
Use Vertipaq Analyzer, eg in Dax Studio.
Model Metrics
DAX Studio has the ability to analyze your data model and report on the memory usage. This can be extremely helpful when performance tuning as the more memory your data structures require, the slower they are to both process and query.