I'll give it a go. The maximum prison sentence depends on the offence, unless you mean in the literal sense. In which case, an indeterminate sentence in theory is the longest custodial sentence because no specific time is set by the court, otherwise it would be a 'life' sentence where the court can impose a minimum term until the offender can be considered for parole or a 'whole life' sentence where the person is never to be considered for realease.

Not sure if Scottish law is different.

The Sheriffs courts has no direct equivalent in the English legal system. It is above the Justice of the Peace Courts which hear summary cases and so are equivalent to the English magistrate’s courts but it is below the High Court of Justiciary which is the Scottish equivalent of the Crown Court. It can either sit as a solemn court with a Scottish type jury (of 15 jurors) or as a summary court with no jury. The maximum penalty that may be imposed in summary cases is 12 months' imprisonment or a £10,000 fine, in solemn cases 5 years' imprisonment or an unlimited fine. A higher sentence in solemn cases may be imposed upon reference to the High Court of Justiciary.