Hello,
Please note that the allowed values when applying height to tr tags (as explained on https://www.phpdocx.com/htmlapi-documentation/html-standard/insert-table-Word-document-with-HTML), are the following:
height length: em, pt, px
You need to use a fixed size to apply a tr height.
About your question, there's no way to prevent repeating a background image, MS Word repeats it automatically. You can get section sizes using the Indexer class (https://www.phpdocx.com/api-documentation/docxutilities/indexer-parse-word-documents-with-PHP). For your reference A4 has the following size:
'width' => '11906',
'height' => '16838',
set as twentieths of a point (https://www.phpdocx.com/documentation/cookbook/word-units-measure).
HTML Extended, available in Premium licenses, includes the phpdocx_image tag (https://www.phpdocx.com/htmlapi-documentation/html-extended/insert-image-Word-document-with-HTML) that supports extra options such as relativeToHorizontal, relativeToVertical, textWrap and others). This tag may be useful for your needs.
Regards.