Transaction's maximum size
There is a maximum standard transaction size since Bitcoin 0.8.2 of 100k per transaction.

There are a number of other limits that influence the validation and propagation of a transaction though. Specifically:


A block is limited to 20000 signature verifications.
The block itself can't be larger than 1Mb.
The standard Bitcoin client (Bitcoin Core / bitcoind) will refuse to relay transactions flagged as dust.
Enough fee should be included (0.0001 BTC/kb)