Complex sentence: The size of the spread is usually denominated in basis points (bps): that is when T-Bills trade at 5.10% and ED trades at 5.50%, the TED spread is 40bps.
Split into simpler sentences: The size of the spread is usually denominated in basis points (bps). For example, if the T-Bill rate is 5.10% and ED trades at 5.50%, the TED spread is 40bps.