Both opcode size limit (if you don't use taproot) and block size if you do use taproot.

Andrew Poelstra later clarifies

Aside from limits on transaction size, post-Taproot script can verify a
trace of any program execution, as long as the individual elements it is
operating on fit into 4-byte CScriptNums. You can therefore implement
SHA2, ECDSA, etc., and reconstruct the pattern of SIZE elements by
feeding in transaction data. Which of course can then be arbitrarily
constrained.

Probably actually doing this would take more than 4 megs of script and
you would need to use some sort of BitVM tricks and the whole thing
might not work. But this was my point in saying that "only the script
limits are stopping us from having covenants".

And pre-Taproot we have only 201 opcodes so of course this is all
totally out of the question :) but plausibly we could make a copy of the
Lamport signature in a Taproot output and then use non-equivocation
slashing conditions to somehow make things work.

You can read the whole thread here: https://groups.google.com/g/bitcoindev/c/mR53go5gHIk/m/csQn7IdEAgAJ?pli=1