mjhjmtu

Chopin wrote four "grandes valses brillantes" (Opera 18, 34) and Beethoven wrote a sonata (Opus 13) that the publisher called a "grande sonate pathétique".

What is the sense of the word "grand" in these titles?

(I'm guessing it doesn't refer to the duration of the piece, like it may well do for Beethoven's Opus 7.)

Grand in this context just means big or large (large-scale). In the two instances you give, these are French descriptors: the Beethoven example roughly translates as "large-scale, sad sonata"; the Chopin "large-scale, shining waltzes". Beethoven would use German in his later piano sonatas, rather than the more fashionable French, e.g. Piano Sonata No. 29 in B♭ major, Op. 106 (aka The Hammerklavier Sonata) has this descriptor on the title page: "Große Sonate für das Hammerklavier" (Large-scale sonata for the pianoforte).

The usage implies a grandiosity of size & scale, but also of expression & scope. The instruments & techniques used by Beethoven, & Chopin after, were capable of more varied & dramatic expression than the those of their forebears, further the Romantic aesthetic that these composers represent in their own ways was an aesthetic of grand gestures & expression when contrasted with the preceding Classical tradition.