Assuming that a subordination strategy is adopted, and the balance sheet is set to shrink passively and predictably once reinvestment ceases or is phased out, there is some uncertainty around the size of the balance sheet when it returns to normal, which the Committee has described as “no larger than necessary for the efficient and effective implementation of monetary policy."
There are good reasons to expect a normalized balance sheet to be considerably smaller than its current size but larger than its pre-crisis level. Most obviously, trend growth in the demand for currency gradually pushes up the size of the balance sheet over time, but there are also other reasons to expect the post-crisis new normal to be larger than pre-crisis levels.