How long is too long for the pipes from the inside to the outside unit?

Or how short should they be before you have to take them into account in deciding on how many BTU you need?

It will vary from manufacturer to manufacturer as to their recommendation for shortest and longest length freon piping for a typical home split AC, but it's usually around 1.5M minimum to around 20 to 25M maximum. A new unit will usually come precharged based on the assumption the freon pipes will be approx 5M in length with instructions on how much additional freon to add per meter additional length beyond the normal. BTU size needs to be based primarily on the size of the area you are cooling (but other factors come in to play also like number windows, number of people, room use, etc); not how long your freon pipes are between the inside and outside units.

pipe lengths affect efficiency slightly Pib. capacity drops 6-8% every 10 meters exceeding 5m.

Here's a page from a Mitsubishi AC manual which may be helpful. One diagram deals with a standard installation with pipes no longer that 5M...another one when using pipes up to approx 20M in length/elevation to include how much additional freon to add per meter when exceeding 5M. Also talks electrical wiring size. Not the easiest page to understand in terms of how they intermix length and elevation....but elevation between the inside and outside unit is a definite consideration since you are pumping freon around.