Ah is just a measure of the total storage in a battery. A higher number is useful, especially if you have a lot of accessories connected, you do short journeys or you leave the bike parked for long periods. If the bike is used frequently for decent length journeys so topped up frequently, a lower Ah rating is okay.
The other measure is CCA (cold cranking amps). This is a measure of the battery efficiency when cold. A bigger number is useful if the bike is used in winter.
Then you need to choose between a wet battery and the more recent developments, gel, AGM or lithium. AGM is my preference.
The Motorworks page gives a lot of different options at
https://www.motorworks.co.uk/catalogsearch/result/?q=+r100/7+battery - the physical size and Ah vary considerably
Tayna is more specific
https://www.tayna.co.uk/motorcycle-batteries/f/bmw/r100-7-76-84/ - note the physical size is pretty consistent
Tayna is my preferred battery supplier. Of those listed, I would ignore the wet batteries although they are a lot cheaper and your OEM battery from the factory would have been wet. The wet CCA is very low. I like Exide batteries and it is 30% cheaper than the lower spec Yuasa and half the price of the Yuasa battery with special gold lettering and 500CCA.