About 1.5 cm. It looks a lot like one Ken-ichi found in 2012 also at Pillar Point (http://www.inaturalist.org/observations/160939) -- which no one would confirm! It is muddy-looking - and branchial plume is the body color - but there are black tips on the papillae and there's a black line at the margin. Its rhinophores look A. rhodoceras-ish, just muted colors.
See Gary MacDonald's page: http://people.ucsc.edu/~mcduck/DreamWeaver/Mollusca/Nudibranchs/Acanthodoris_rhodoceras/Acanthodoris_rhodoceras.html