Trust size is effectively the amount of money raised in the IPO for the SPAC that is held in the escrow account used to merge with a future company.
The Net Asset Value is clearly found in the S-1 SEC filing for every SPAC. For just about every SPAC except for Pershing Square Tontine Holdings, the NAV is 10 USD.
After the initial pop when a target company is announced, either through a rumour (typically on Bloomberg), a formal letter-of-interest (LOI), or a definitive agreement (DA), that is when you need to do your homework and decide if the target company is one you want to keep in the long run. There is typically a drop in stock price after the initial pop, and then if the target company is perceived as desirable, a steady climb back up leading to the merger. After the merge, most SPACs tend to drop in price again. It is not recommended by most SPAC investors to hold through the merge, unless you are convinced with a company's prospects.