Using many resistors to scale down AC voltage
With resistor values that high the power dissipation will almost certainly not be a problem. Voltage capability will be though.
The voltage rating of a resistor depends largely on its physical construction. Large through hole resistors are often rated for 250V or higher. The power rating will also depend upon the physical size so they will tend to go hand-in-hand.
Some resistors designed for high-voltage can withstand thousands of volts. They will be physically large and expensive though.
At the other extreme small surface mount resistors (0603, 0402) will usually have a rating of only 50V.
You will need to select the quantity of resistors that will meet your upper voltage limit. For example if the the resistors tolerate 250V and you have a 1kV peak input you need at least 4 resistors - maybe 5 or 6.
If you select 5 resistors and each one is 100 MOhm the current will 2 microamps. The total power dissipation will only be 2mW. This is only 400uW per resistor - insignificant and not an issue.
You don't say what bandwidth you require from the attenuator. With a value of 100MOhm per resistor, you will almost certainly need to put balancing capacitors across each resistor because of the uncertainty of the parasitic capacitances.
A typical capacitance for a 0.25W through-hole resistor might be 0.25pF plus another 0.5pF for the via and trace on the PCB. In general, the approach is to swamp that with a real capacitor that is much larger, eg 5pF or 10pF. Some designers have built this capacitor into the PCB, but this will require some experimentation and care.
In this case, the bottom resistor of the will need to be ~1.25MOhm to get the 2.5V output that you need. The capacitance across it should be such that you achieve the same time constant as the other RC combinations so the attenuation of the entire divider is independent of frequency.
To match the 10pF 100MOhm combination on each resistor will require ~800pF across the bottom resistor. You may need a trimmer capacitance to adjust the compensation (just like on a scope probe).
As @Sphero Pefhany comments even 100MOhm is a rather high value and out of the normal range of resistors. Unless you really need that high an input resistance using resistors of 10MOhms or lower will be much cheaper and provide you with better selection and availability. The power dissipation will increase and although not likely to be an issue in this example might be in others.