Small size ultra short waves (UKW) emitter
I can give you a few pointers as to whether this might be practical. 

It is generally accepted that the power needed at ambient temperature to adequately receive data is -154 dBm + 10 log\$_{10}\$(data rate). So, if your data rate is 1 bit(s) per second, a receiver should just about be able to work with an input power from the antenna of -154 dBm. If the data rate was 10 bits per second then the power needed would be -144 dBm.

However, this isn't an unhindered transmission through free space so a general rule of thumb is to add back another 30 dB of power so, you might get away (most of the time) with about -124 dBm.

Of course you need to design the receiver so that it had fairly substantial band-pass filtering and a 1 bit per second data rate would require a 1 Hz filter as a general rule. This does of course mean that your transmitter has to transmit a carrier that is substantially better than 1 Hz accurate or the system just won't work.

So, you design a receive band pass filter that is wide enough to cover the drift of the transmitter and this might mean the wider bandwidth accepts much more noise interference. This is where the formula I wrote above comes into play. It's an empirical formula that assumes you do some good filtering in your reciever and that the transmit carrier remains substantially within the range of that receive filter.

So, realistically you are looking to work with a power that might be about -110 dBm to get good data. 

There is another formula invented by Harald Friis that describes how power is thinned out with distance and it also brings in the transmission frequency: -

Loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where f is MHz and d is kilometres.

So at 170 MHz the attenuation over a 100 km distance is 32.45 dB + 44.6 dB + 40 dB = 117 dB. There is another assumption here; the antennas used are undirectional and, in a lot of practical situations you can use somewhat directional antennas.

Amended answer due to math mistake

So you might be able to get a loss of about 113 dB using a dipole antenna and this means you need to transmit -110 dBm + 113 dB = 3 dBm or 2 mW. This is feasible from a small battery and is certainly feasible with a module that is 5 cm x 5 cm.

However, at VHF frequencies, you get no reflections from the ionosphere so you need a line-of-site communication and this means mounting transmit antenna and receive antenna very high up. For instance, using THIS calculator you would need 150 metres height at both ends to get line of sight.