How to determine the physical size of SMD capacitor.

[Kokoriantz]

I need smd 3.3nF charging 48v with 1.25A pulse. Should I use small size 0608 or big size, 1206 more dissipation?

[Andree Henkel]

I=C*dU/dt; I=1,25A; C=3,3nV
gives Voltage Slope 379V/µs across the cap
Are you sure?

H´m current ratings are not typically to be found in datasheets for chipceramic caps of lower nF Value

so good luck

you may need some caps of smaller value in parallel of smaller size, probably preferable to one larger

reverse geometrie is probably preferable for lower ESL and ESR, as you enter RF territory regarding Voltage Slope
even better suitable could be X2Y type like:
https://www.digikey.de/de/products/filter/keramikkondensatoren/60?s=N4IgjCBcpgnAHLKoDGUBmBDANgZwKYA0IA9lANogAMIAugL7EC0AzMiGpAC4BOArkVIVwNBswjQOGHAWJlIlAOxUkxRWABMdRiCYAWdpyx5B8ygFYWsRSGIa9ih9vr0gA
there are comparisons in decoupling applications comparing standard, reverse geometriy and X2Y type.
bottom line for suitable decoupling of like a FPGA power rail x2y can do it with fewest caps and area, standard needs most caps and area and reversed geometry is in between.

what do you actually want to achieve? Decoupling or what?

[Kokoriantz]

It is a 2.2uH inductor that gets charged with 2.5A by a pair of 3.3nF on vcc and ground. The fly wheel current in the inductor is maintained by a pair of diodes.
The frequency is 300khz.

[Terry Bites]

The limiting factor will probably be the current handling of the interconnect.
Dielectric dissipation could be as high as 1-2% for high K capacitors.