Namcot-106 (mapper 19) banks are 8KB in size ($8000, $A000, $C000 are swappable, swap them by writing to $E000, $E800, $F000 respectively).
I forget what it was in Bakabon, but in Kaiju Monogatari, it was $70+bank. To find what the game is using, just set a breakpoint for the bankswap registers in FCEUX.
The bank value written was ANDed by the last bank value. For example, Kaiju had 128KB PRG, so the last bank was $0F, so like if the game wrote $78 to the register, it'd be $78 AND $0F = $08. Which is why I had to expand at the beginning (256KB = 32 banks), because $78 AND $1F = $18, so I'd want the original bank $08 to become expanded bank $18.
I don't know if Juvei Quest uses the Namcot-106 built-in RAM for battery-backup purposes like Kaiju did, or if it used normal SRAM ($6000-7FFF). One site I looked at says it does use normal SRAM.
(Namcot-106 chip's RAM: $F800 is used to select the RAM address. Low 7 bits select the address (total 128 bytes), high bit chooses to disable (0) /enable (1) auto-increment upon read/write. Read/write $4800 to access RAM.)

I did some work on this one as one of my future projects... As I remember, the banks there are placed in the same way as in Dream Master, they are calculated via a rather complicated formula and go in reverse order. First - new banks, then - the old ones in an expanded rom.

I'm pretty sure it's a good idea to avoid non-power of two PRG sizes.
In other words, if you increase PRG ROM's size, double it.

Oh yeah of course that is right. I didn't know the game used Character ROM, as not all NES games use that.