Component specifications needed for a high voltage front-end
Using 0603 resistors for Rhi is probably not adequate.  For mains voltage you probably want 1210 or 2512 sizes.  Rlo, Rflt, Cflt will see much less voltage, so 0603 is probably adequate for those.
To determine the what package you need for a particular voltage you need to at least look at the following.

Manufacturer Rating.  You need to consider what the manufacturer datasheet for a particular part series says is the max operating voltage.  For example, page 1 of the Vishay CRCWe3 resistor datasheet gives the following voltage ratings for different package sizes.

https://www.vishay.com/docs/20035/dcrcwe3.pdf

0402 40V
0603 75V
0805 150V
1206 200V
2010 400V
2512 500V


Certification Standards.  You need to consider what IPC or UL standards are you trying to meet regarding creepage and clearance on the PCB.  For that you need to look at the distance between the pads in the manufacturer suggested footprint (which is also in the datasheet).  For example, page 9 of the Vishay CRCWe3 resistor datasheet gives the following clearances (dimension G) for different package sizes.


0402 0.45 mm, IPC2221B Table 6-1, up to 30V
0603 0.75 mm, IPC2221B Table 6-1, up to 150V
0805 1.00 mm, IPC2221B Table 6-1, up to 150V
1206 1.50 mm, IPC2221B Table 6-1, up to 300V
2010 1.70 mm, IPC2221B Table 6-1, up to 300V
2512 4.75 mm, IPC2221B Table 6-1, up to 500V

Consult the relevant standards for full details on spacing requirements.
You may also try online tools such as...
http://www.creepage.com/

Pulsed voltage rating.
a. The pulsed voltage rating is usually given in the datasheet as a graph of peak voltage vs time.
b. The pulsed rating should exceed what you expect to see following your front end transient voltage suppressor.
c.  For example, page 5 of the CRCWe3 datasheet shows that the 2512 package can withstand a pulse of 1600V for 1ms or less.
d.  Build in lots of margin.  Its worth the extra $0.25 to prevent someone from being injured or killed when a fault occurs.

SEARCH TOOLS
Some distributors such as Mouser let you select voltage rating as a criteria when searching for resistors.
https://www.mouser.com/Passive-Components/Resistors/SMD-Resistors-Chip-Resistors/Thick-Film-Resistors/_/N-7h7yz