My answer will rely largely on the Feb 2019 Monetary Policy Report (summary, full report PDF). From p. 41:
The size of the Federal Reserve’s balance sheet increased from \$900 billion at the end of 2006 to about \$4.5 trillion at the end of 2014
Important note: Your second quote, which seems to be from The Associated Press, claims that Powell "has said that" the Fed will have "a balance sheet probably no lower than around \$3.5 trillion".
However, I am unable to find anywhere else any such quote or statement by a Fed member that explicitly states this \$3.5 trillion figure.
This though is the estimate that has been floating around for at least some months now — e.g. in Jan 2019, UBS economist Seth Carpenter predicted this particular figure for mid-2020. So, it is possible that the AP writer simply accidentally slipped it in.
What Powell/the Fed do say is this (p. 43):
the longer-run size of the Federal Reserve’s balance sheet will be considerably larger than before the crisis.
The reasons are:
(1) projected trend growth for currency in circulation, (2) the Committee’s decision to continue operating with ample reserves, and the higher levels for (3) the TGA, (4) the foreign repo pool, and (5) DFMU balances (I added the numbering).
This tends to rise in line with nominal GDP and has risen from around \$800B at end 2006 to around \$1.7T now (data).
(2) "Floor system" or "abundant reserves system"
Pre-crisis, bank reserves held at the Fed were negligible (data). In 2014, they peaked at around \$2.8T and have now fallen to around \$1.7T.
At its January 2019 meeting, the Federal Open Market Committee decided that it would continue to implement monetary policy in a regime with an ample supply of reserves, which is often called a “floor system” or an "abundant reserves system." Going forward, the banking system’s overall demand for reserve balances and the Committee’s judgment about the quantity that is appropriate for the efficient and effective implementation of monetary policy will determine the longer-run level of reserve balances. Although the level of reserve balances that banks will eventually demand is not yet known with certainty, it is likely to be appreciably higher than before the crisis (p. 42).
(3) Treasury General Account (TGA)
Before 2008, the Treasury targeted a steady, low balance of $5 billion in the TGA on most days ... In May 2015, the Treasury announced its intention to hold in the TGA a level of cash generally sufficient to cover one week of outflows, subject to a minimum balance objective of roughly \$150 billion. Since this policy change, the TGA balance has generally been well above this minimum; at the end of 2018, it was about \$370 billion, or nearly 2 percent of GDP. The current policy helps protect against the risk that extreme weather or other technical or operational events might cause an interruption in access to debt markets and leave the Treasury unable to fund U.S. government operations (p. 42)
(4) Foreign repo pool
The foreign repo pool has grown from an average level of around \$30 billion before the crisis to a current average of about \$250 billion (p. 43)
(5) "Other deposits"
“other deposits” with the Federal Reserve Banks have also risen steadily over recent years, from less than \$1 billion before the crisis to about \$80 billion at the end of 2018. Although “other deposits” include balances held by international and multilateral organizations, government-sponsored enterprises, and other miscellaneous items, the increase has largely been driven by the establishments of accounts for DFMUs. DFMUs provide the infrastructure for transferring, clearing, and settling payments, securities, and other transactions among financial institutions. The Dodd-Frank Wall Street Reform and Consumer Protection Act provides that DFMUs—those financial market utilities designated as systemically important by the Financial Stability Oversight Council—can maintain accounts at the Federal Reserve and earn interest on balances maintained in those accounts.
Again, I am unable to find \$3.5T explicitly stated anywhere by the Fed/Powell. But at the current balance sheet of about \$4T, current pace of unwinding of about \$30–50B per month, and expected cessation of unwinding some time in mid-2020, \$3.5T is a nice round number that seems plausible and easy to quote.