The error message saying "x is too large to fit" means that the report hasn't been told how to split that section up so it'll never fit on any single page.
When the report engine paginates a report it has to work out what bands will fit on the current page, taking into account things like, does the report have a title band, is the title band on this page, does the page header fit, after the page header how much space is left, is there are page footer that takes us any space that reduces the amount of available space, what are the printer's physical printer boundaries for the page size (A4) that have to be considered. In short, after report title, page header, page footer, report summary, group header/footers how much space for "details" does this page have and it has to asses that for each page in turn.
As it performs all these pagination calculations it knows how much space it has left and will then look at the details band or bands (or any band type for that matter) to determine if there is space for the band or bands. As it looks through the bands if there is not enough space on the current page then it flags that band for going onto the next page and moves on. It looks at the next page and starts it's calculations for that page, then assesses whether that band will fit on the next page.
If the report engine gets to a situation where the band won't fit on the current page and then the calculations say it won't fit on the next page either then to save itself from getting into an infinite loop it displays that error message because, in essence, it can't proceed as the details will not fit on any page, it wouldn't be any use trying the next page or the next page, it has hit a failure state.
In your case, you only have one details band and Detail 1 is so long > 50cm that it'll never fit on any single page of A4 so it triggers the error for the reasons explained above.
What I suggest you do is split your details into multiple bands, I can't tell you exactly how many to do, or how to arrange the report elements into those bands BUT you do need to split them up into sections. Since your report does appear to have some very obvious sections, I'd recommend splitting them up into those sections if that makes logic sense to be kept together, perhaps in the 1.1, 1.2, 1.3 sections you've got as text labels.
To do that, go to the Optional Bands section of the Report Properties and add a number of additional Details bands, I imagine you'll maybe want more than 10, maybe even more than 20, such is the busy nature of your report. Then start dragging the report elements into the bands as makes sense for your layout.
Once you've got multiple bands, and no single band is ever taller than can fit on an A4 page (given page headers, page footers, physical printer boundaries etc) then the report will be able to bump any details band that won't fit onto the current page, onto the next page and you'll stop that error occurring. It'll still be your responsibility to get the report looking OK but your #1 issue right now is to get the information spread out between multiple bands to give the report engine a fighting chance of producing any pages.
Hope that helps.
Paul