I'd like to do an fft~ on a signal, but I want an analysis size of more than 64 samples. I know I can change the block size with block~, but then the signal isn't compatible with the sound card and won't be audible. How else can I achieve this?

Make a subpatch (like [pd $0-fft-analyser] for example), put the [block~ ...] and [fft~] etc in there, and connect to the rest of the patch using [inlet~] and [outlet~]. The [block~] will affect only the subpatch containing it, so leave the [adc~] and [dac~] outside this subpatch.