Set `maxTxSize` to a sensible value in conformance tests

UTXO rule in ledger checks that the size of a transaction doesn't exceed `maxTxSize` pparam. However, this check does not seem to be implemented in the Agda spec, so until this happens, we have to generate transactions that won't cause this predicate failure.
The way it's been done at the moment in the UTXO conformance test is to set the pparam to be the size of the generated transaction, which is a workaround to prevent the maxSize being exceeded and hence the test from failing.

Once the check is implemented in the spec, we should set this value in a more intelligent way: perhaps in different test cases to:
* test that the check is indeed happening as it should (when set to something small
* for it to be more realistic, otherwise